Added F.54 to never capture this implicitly

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hsutter 2016-01-11 11:00:27 -08:00
parent f52cc108e0
commit 3668c8c491

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@ -1832,6 +1832,7 @@ Other function rules:
* [F.51: Prefer overloading over default arguments for virtual functions](#Rf-default-args)
* [F.52: Prefer capturing by reference in lambdas that will be used locally, including passed to algorithms](#Rf-reference-capture)
* [F.53: Avoid capturing by reference in lambdas that will be used nonlocally, including returned, stored on the heap, or passed to another thread](#Rf-value-capture)
* [F.54: If you capture `this`, capture all variables explicitly (no default capture)](#Rf-this-capture)
Functions have strong similarities to lambdas and function objects so see also Section ???.
@ -3053,6 +3054,51 @@ Pointers and references to locals shouldn't outlive their scope. Lambdas that ca
???
### <a name="Rf-this-capture"></a>F.54: If you capture `this`, capture all variables explicitly (no default capture)
##### Reason
It's confusing. Writing `[=]` in a member function appears to capture by value, but actually captures data members by reference because it actually captures the invisible `this` pointer by value. If you meant to do that, write `this` explicitly.
##### Example
class myclass {
int x = 0;
// ...
void f() {
int i = 0;
// ...
auto lambda = [=]{ use(i,x); }; // BAD: "looks like" copy/value capture
x = 42;
lambda(); // calls use(42);
x = 43;
lambda(); // calls use(43);
// ...
auto lambda2 = [=,this]{ use(i,x); }; // BAD: not much better, and confusing
auto lambda3 = [&,this]{ use(i,x); }; // BAD: not much better, and confusing
auto lambda4 = [&]{ use(i,x); }; // BAD: if lambda2 leaves the scope, it contains a dangling reference to the this parameter from this function's invocation
// ...
auto lambda5 = [i,this]{ use(i,x); }; // ok, most explicit and least confusing
// ...
}
};
##### Note
This is under active discussion in standarization, and may be addressed in a future version of the standard by adding a new capture mode or possibly adjusting the meaning of `[=]`. For now, just be explicit.
##### Enforcement
* Flag any lambda capture-list that specifies a default capture and also captures `this` (whether explicitly or via default capture)
# <a name="S-class"></a>C: Classes and Class Hierarchies
A class is a user-defined type, for which a programmer can define the representation, operations, and interfaces.