mirror of
https://github.com/isocpp/CppCoreGuidelines.git
synced 2024-03-22 13:30:58 +08:00
Gabriel Dos Reis
GitHub
commit
db029855dd
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@ -3142,10 +3142,10 @@ Here on one popular implementation I got the output:
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I expected that because the call of `g()` reuses the stack space abandoned by the call of `f()` so `*p` refers to the space now occupied by `gx`.
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Imagine what would happen if `fx` and `gx` were of different types.
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Imagine what would happen if `fx` or `gx` was a type with an invariant.
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Imagine what would happen if more that dangling pointer was passed around among a larger set of functions.
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Imagine what a cracker could do with that dangling pointer.
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* Imagine what would happen if `fx` and `gx` were of different types.
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* Imagine what would happen if `fx` or `gx` was a type with an invariant.
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* Imagine what would happen if more that dangling pointer was passed around among a larger set of functions.
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* Imagine what a cracker could do with that dangling pointer.
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Fortunately, most (all?) modern compilers catch and warn against this simple case.
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@ -3400,7 +3400,7 @@ There is not a choice when a set of functions are used to do a semantically equi
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##### See also
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[Default arguments for virtual functions](#Rh-virtual-default-arg}
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[Default arguments for virtual functions](#Rh-virtual-default-arg)
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##### Enforcement
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@ -7368,7 +7368,7 @@ Wrap a `union` in a class together with a type field.
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The soon-to-be-standard `variant` type (to be found in `<variant>`) does that for you:
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variant<int,double> v;
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variant<int, double> v;
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v = 123; // v holds an int
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int x = get<int>(v);
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v = 123.456; // v holds a double
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@ -7420,19 +7420,19 @@ Saving programmers from having to write such code is one reason for including `v
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int Value::number() const
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{
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if (type!=Tag::number) throw Bad_entry{};
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if (type != Tag::number) throw Bad_entry{};
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return i;
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}
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string Value::text() const
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{
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if (type!=Tag::text) throw Bad_entry{};
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if (type != Tag::text) throw Bad_entry{};
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return s;
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}
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void Value::set_number(int n)
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{
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if (type==Tag::text) {
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if (type == Tag::text) {
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s.~string(); // explicitly destroy string
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type = Tag::number;
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}
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@ -7441,7 +7441,7 @@ Saving programmers from having to write such code is one reason for including `v
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void Value::set_text(const string& ss)
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{
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if (type==Tag::text)
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if (type == Tag::text)
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s = ss;
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else {
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new(&s) string{ss}; // placement new: explicitly construct string
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@ -7451,12 +7451,12 @@ Saving programmers from having to write such code is one reason for including `v
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Value& Value::operator=(const Value& e) // necessary because of the string variant
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{
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if (type==Tag::text && e.type==Tag::text) {
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if (type == Tag::text && e.type == Tag::text) {
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s = e.s; // usual string assignment
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return *this;
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}
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if (type==Tag::text) s.~string(); // explicit destroy
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if (type == Tag::text) s.~string(); // explicit destroy
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switch (e.type) {
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case Tag::number:
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@ -7472,7 +7472,7 @@ Saving programmers from having to write such code is one reason for including `v
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Value::~Value()
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{
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if (type==Tag::text) s.~string(); // explicit destroy
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if (type == Tag::text) s.~string(); // explicit destroy
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}
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##### Enforcement
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@ -8460,14 +8460,14 @@ Any type (including primary template or specialization) that overloads unary `*`
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##### Example
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// use Boost's intrusive_ptr
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#include <boost/intrusive_ptr.hpp>
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#include<boost/intrusive_ptr.hpp>
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void f(boost::intrusive_ptr<widget> p) // error under rule 'sharedptrparam'
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{
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p->foo();
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}
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// use Microsoft's CComPtr
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#include <atlbase.h>
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#include<atlbase.h>
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void f(CComPtr<widget> p) // error under rule 'sharedptrparam'
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{
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p->foo();
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@ -9168,7 +9168,7 @@ Reuse of a member name as a local variable can also be a problem:
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void S::f(int x)
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{
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m=7; // assign to member
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m = 7; // assign to member
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if (x) {
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int m = 9;
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// ...
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@ -9487,7 +9487,9 @@ For containers, there is a tradition for using `{...}` for a list of elements an
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Initialization of a variable declared using `auto` with a single value, e.g., `{v}`, had surprising results until recently:
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auto x1 {7}; // x1 is an int with the value 7
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auto x2 = {7}; // x2 is an initializer_list<int> with an element 7 (this will will change to "element 7" in C++17)
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// x2 is an initializer_list<int> with an element 7
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// (this will will change to "element 7" in C++17)
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auto x2 = {7};
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auto x11 {7, 8}; // error: two initializers
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auto x22 = {7, 8}; // x2 is an initializer_list<int> with elements 7 and 8
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@ -9776,7 +9778,7 @@ Requires messy cast-and-macro-laden code to get working right.
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##### Example
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#include <cstdarg>
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#include<cstdarg>
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// "severity" followed by a zero-terminated list of char*s; write the C-style strings to cerr
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void error(int severity ...)
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@ -10364,7 +10366,7 @@ A key example is basic narrowing:
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double d = 7.9;
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int i = d; // bad: narrowing: i becomes 7
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i = (int)d; // bad: we're going to claim this is still not explicit enough
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i = (int) d; // bad: we're going to claim this is still not explicit enough
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void f(int x, long y, double d)
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{
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@ -10511,7 +10513,7 @@ Consider keeping previously computed results around for a costly operation:
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class Cache { // some type implementing a cache for an int->int operation
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public:
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pair<bool,int> find(int x) const; // is there a value for x?
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pair<bool, int> find(int x) const; // is there a value for x?
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void set(int x, int v); // make y the value for x
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// ...
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private:
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@ -10525,7 +10527,7 @@ Consider keeping previously computed results around for a costly operation:
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auto p = cache.find(x);
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if (p.first) return p.second;
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int val = compute(x);
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cache.set(x,val); // insert value for x
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cache.set(x, val); // insert value for x
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return val;
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}
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// ...
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@ -10543,7 +10545,7 @@ To do this we still need to mutate `cache`, so people sometimes resort to a `con
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auto p = cache.find(x);
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if (p.first) return p.second;
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int val = compute(x);
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const_cast<Cache&>(cache).set(x,val); // ugly
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const_cast<Cache&>(cache).set(x, val); // ugly
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return val;
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}
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// ...
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@ -10561,7 +10563,7 @@ State that `cache` is mutable even for a `const` object:
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auto p = cache.find(x);
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if (p.first) return p.second;
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int val = compute(x);
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cache.set(x,val);
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cache.set(x, val);
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return val;
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}
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// ...
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@ -10939,12 +10941,15 @@ The build-in array uses signed types for subscripts.
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This makes surprises (and bugs) inevitable.
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int a[10];
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for (int i=0; i<10; ++i) a[i]=i;
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for (int i=0; i < 10; ++i) a[i]=i;
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vector<int> v(10);
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for (int i=0; v.size()<10; ++i) v[i]=i; // compares signed to unsigned; some compilers warn
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// compares signed to unsigned; some compilers warn
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for (int i=0; v.size() < 10; ++i) v[i]=i;
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int a2[-2]; // error: negative size
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vector<int> v2(-2); // OK, but the number of ints (4294967294) is so large that we should get an exception
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// OK, but the number of ints (4294967294) is so large that we should get an exception
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vector<int> v2(-2);
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##### Enforcement
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@ -11190,7 +11195,7 @@ Because a design that ignore the possibility of later improvement is hard to cha
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From the C (and C++) standard:
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void qsort (void* base, size_t num, size_t size, int (*compar)(const void*,const void*));
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void qsort (void* base, size_t num, size_t size, int (*compar)(const void*, const void*));
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When did you even want to sort memory?
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Really, we sort sequences of elements, typically stored in containers.
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@ -11200,7 +11205,10 @@ This implies added work for the programmer, is error prone, and deprives the com
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double data[100];
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// ... fill a ...
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qsort(data,100,sizeof(double),compare_doubles); // 100 chunks of memory of sizeof(double) starting at address data using the order defined by compare_doubles
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// 100 chunks of memory of sizeof(double) starting at
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// address data using the order defined by compare_doubles
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qsort(data, 100, sizeof(double), compare_doubles);
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From the point of view of interface design is that `qsort` throws away useful information.
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@ -11209,13 +11217,15 @@ We can do better (in C++98)
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template<typename Iter>
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void sort(Iter b, Iter e); // sort [b:e)
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sort(data,data+100);
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sort(data, data + 100);
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Here, we use the compiler's knowledge about the size of the array, the type of elements, and how to compare `double`s.
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With C++11 plus [concepts](#???), we can do better still
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void sort(Sortable& c); // Sortable specifies that c must be a random-access sequence of elements comparable with <
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// Sortable specifies that c must be a
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// random-access sequence of elements comparable with <
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void sort(Sortable& c);
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sort(c);
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@ -11224,7 +11234,8 @@ In this, the `sort` interfaces shown here still have a weakness:
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They implicitly rely on the element type having less-than (`<`) defined.
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To complete the interface, we need a second version that accepts a comparison criteria:
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void sort(Sortable& c, Predicate<Value_type<Sortable>> p); // compare elements of c using p
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// compare elements of c using p
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void sort(Sortable& c, Predicate<Value_type<Sortable>> p);
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The standard-library specification of `sort` offers those two versions,
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but the semantics is expressed in English rather than code using concepts.
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@ -11267,7 +11278,7 @@ Don't let bad designs "bleed into" your code.
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Consider:
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template <class ForwardIterator, class T>
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bool binary_search (ForwardIterator first, ForwardIterator last, const T& val);
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bool binary_search(ForwardIterator first, ForwardIterator last, const T& val);
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`binary_search(begin(c),end(c),7)` will tell you whether `7` is in `c` or not.
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However, it will not tell you where that `7` is or whether there are more than one `7`.
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@ -11283,13 +11294,13 @@ needed information back to the caller. Therefore, the standard library also offe
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However, `lower_bound` still doesn't return enough information for all uses, so the standard library also offers
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template <class ForwardIterator, class T>
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pair<ForwardIterator,ForwardIterator>
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equal_range (ForwardIterator first, ForwardIterator last, const T& val);
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pair<ForwardIterator, ForwardIterator>
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equal_range(ForwardIterator first, ForwardIterator last, const T& val);
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`equal_range` returns a `pair` of iterators specifying the first and one beyond last match.
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auto r = equal_range(begin(c),end(c),7);
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for (auto p = r.first(); p!=r.second(), ++p)
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auto r = equal_range(begin(c), end(c),7);
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for (auto p = r.first(); p != r.second(), ++p)
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cout << *p << '\n';
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Obviously, these three interfaces are implemented by the same basic code.
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@ -12069,7 +12080,7 @@ A `thread` that has not been `detach()`ed when it is destroyed terminates the pr
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##### Enforcement
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* Flag `join's for `raii_thread`s ???
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* Flag `join`s for `raii_thread`s ???
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* Flag `detach`s for `detached_thread`s
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@ -13957,7 +13968,7 @@ It also avoids brittle or inefficient workarounds. Convention: That's the way th
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};
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Container c(10, sizeof(double));
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((double*)c.elem)[] = 9.9;
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((double*) c.elem)[] = 9.9;
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This doesn't directly express the intent of the programmer and hides the structure of the program from the type system and optimizer.
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@ -15458,7 +15469,7 @@ Variadic templates is the most general mechanism for that, and is both efficient
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##### Enforcement
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* Flag uses of `va_arg` in user code.
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* Flag uses of `va_arg` in user code.
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### <a name="Rt-variadic-pass"></a>T.101: ??? How to pass arguments to a variadic template ???
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@ -15612,7 +15623,7 @@ Often a `constexpr` function implies less compile-time overhead than alternative
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##### Enforcement
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* Flag template metaprograms yielding a value. These should be replaced with `constexpr` functions.
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* Flag template metaprograms yielding a value. These should be replaced with `constexpr` functions.
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### <a name="Rt-std-tmp"></a>T.124: Prefer to use standard-library TMP facilities
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@ -15762,7 +15773,7 @@ Use `!=` instead of `<` to compare iterators; `!=` works for more objects becaus
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// ...
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}
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Of course, range-for is better still where it does what you want.
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Of course, range-`for` is better still where it does what you want.
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##### Example
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@ -15891,7 +15902,7 @@ That subset can be compiled with both C and C++ compilers, and when compiled as
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int* p1 = malloc(10 * sizeof(int)); // not C++
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int* p2 = static_cast<int*>(malloc(10 * sizeof(int))); // not C, C-style C++
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int* p3 = new int[10]; // not C
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int* p4 = (int*)malloc(10 * sizeof(int)); // both C and C++
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int* p4 = (int*) malloc(10 * sizeof(int)); // both C and C++
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##### Enforcement
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@ -16409,7 +16420,7 @@ This slowdown can be significant compared to `printf`-style output.
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##### Example
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cout << "Hello, World!" << endl; // two output operations and a flush
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cout << "hello, World!\n"; // one output operation and no flush
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cout << "Hello, World!\n"; // one output operation and no flush
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##### Note
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@ -16546,9 +16557,9 @@ In particular, the single-return rule makes it harder to concentrate error check
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// requires Number<T>
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string sign(T x)
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{
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if (x<0)
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if (x < 0)
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return "negative";
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else if (x>0)
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else if (x > 0)
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return "positive";
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return "zero";
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}
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@ -16560,12 +16571,12 @@ to use a single return only we would have to do something like
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string sign(T x) // bad
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{
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string res;
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if (x<0)
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if (x < 0)
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res = "negative";
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else if (x>0)
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else if (x > 0)
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res = "positive";
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else
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res ="zero";
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res = "zero";
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return res;
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}
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@ -16577,7 +16588,7 @@ Of course many simple functions will naturally have just one `return` because of
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int index(const char* p)
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{
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if (p==nullptr) return -1; // error indicator: alternatively `throw nullptr_error{}`
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if (p == nullptr) return -1; // error indicator: alternatively "throw nullptr_error{}"
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// ... do a lookup to find the index for p
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return i;
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}
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@ -16587,7 +16598,7 @@ If we applied the rule, we'd get something like
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int index2(const char* p)
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{
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int i;
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if (p==nullptr)
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if (p == nullptr)
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i = -1; // error indicator
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else {
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// ... do a lookup to find the index for p
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@ -16715,9 +16726,9 @@ This technique is a pre-exception technique for RAII-like resource and error han
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void do_something(int n)
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{
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if (n<100) goto exit;
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if (n < 100) goto exit;
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// ...
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int* p = (int*)malloc(n);
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int* p = (int*) malloc(n);
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// ...
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if (some_ error) goto_exit;
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// ...
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