C++ : All About Temporaries

Even the most trivial statements, like A = B, in a computer language may produce temporaries. Moreover, the generation of these temporaries has to be standardized to maintain a language's efficacy. The C++ language is no exception to that rule.

Following is an embellished version of 'The C++ Standard'.


Temporary Objects

1 Temporaries of class type are created in various contexts: binding an rvalue to a reference, returning an rvalue, a conversion that creates an rvalue, throwing an exception, entering a handler, and in some initializations. Even when the creation of the temporary object is avoided, all the semantic restrictions must be respected as if the temporary object was created. [Example: even if the copy constructor is not called, all the semantic restrictions, such as accessibility, shall be satisfied. ]

2 [Example:

class X {
// ...
public:
// ...
X(int);
X(const X&);
˜X();
};

X f(X);

void g()
{
X a(1);
X b = f(X(2));
a = f(a);
}

Here, an implementation might use a temporary in which to construct X(2) before passing it to f() using X’s copy-constructor; alternatively, X(2) might be constructed in the space used to hold the argument. Also, a temporary might be used to hold the result of f(X(2)) before copying it to b using X’s copy-constructor; alternatively, f()’s result might be constructed (directly) in b. On the other hand, the expression a=f(a) requires a temporary for either the argument a or the result of f(a) to avoid undesired aliasing of a. ]

3 When an implementation introduces a temporary object of a class that has a non-trivial constructor, it shall ensure that a constructor is called for the temporary object. Similarly, the destructor shall be called for a temporary with a non-trivial destructor. Temporary objects are destroyed as the last step in evaluating the full-expression (a full-expression is an expression that is not a subexpression of another expression) that (lexically) contains the point where they were created. This is true even if that evaluation ends in throwing an exception.

4 There are two contexts in which temporaries are destroyed at a different point than the end of the full-expression . The first context is when an expression appears as an initializer for a declarator defining an object. In that context, the temporary that holds the result of the expression shall persist until the object’s initialization is complete. The object is initialized from a copy of the temporary; during this copying, an implementation can call the copy constructor many times; the temporary is destroyed after it has been copied, before or when the initialization completes. If many temporaries are created by the evaluation of the initializer, the temporaries are destroyed in reverse order of the completion of their construction.

5 The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object to a subobject of which the temporary is bound persists for the lifetime of the reference except as specified below. A temporary bound to a reference member in a constructor’s ctor-initializer persists until the constructor exits. A temporary bound to a reference parameter in a function call persists until the completion of the full expression containing the call. A temporary bound to the returned value in a function return statement persists until the function exits. In all these cases, the temporaries created during the evaluation of the expression initializing the reference, except the temporary to which the reference is bound, are destroyed at the end of the full-expression in which they are created and in the reverse order of the completion of their construction. If the lifetime of two or more temporaries to which references are bound ends at the same point, these temporaries are destroyed at that point in the reverse order of the completion of their construction. In addition, the destruction of temporaries bound to references shall take into account the ordering of destruction of objects with static or automatic storage duration; that is, if obj1 is an object with static or automatic storage duration created before the temporary is created, the temporary shall be destroyed before obj1 is destroyed; if obj2 is an object with static or automatic storage duration created after the temporary is created, the temporary shall be destroyed after obj2 is destroyed. [Example:

class C {
// ...
public:
C();
C(int);
friend C operator+( const C&, const C& );
˜C();
};

C obj1;
const C& cr = C(16)+C(23);
C obj2;

the expression C(16)+C(23) creates three temporaries. A first temporary T1 to hold the result of the expression C(16), a second temporary T2 to hold the result of the expression C(23), and a third temporary T3 to hold the result of the addition of these two expressions. The temporary T3 is then bound to the reference cr. It is unspecified whether T1 or T2 is created first. On an implementation where T1 is created before T2, it is guaranteed that T2 is destroyed before T1. The temporaries T1 and T2 are bound to the reference parameters of operator+; these temporaries are destroyed at the end of the full expression containing the call to operator+. The temporary T3 bound to the reference cr is destroyed at the end of cr’s lifetime, that is, at the end of the program. In addition, the order in which T3 is destroyed takes into account the destruction order of other objects with static storage duration. That is, because obj1 is constructed before T3, and T3 is constructed before obj2, it is guaranteed that obj2 is destroyed before T3, and that T3 is destroyed before obj1. ]