C + + temporary materialization

Consider the following code:

structS { Public: S (inti) {m=i; }    ~S () {std::cout<<"~s () called \ n"; }    intm;};voidFun (s s) {std::cout<<"Fun called \ n";}intMain () {fun (S) (1));}

S (1) is constructed once, is a nameless temporary object (it is named TMP), and then passed to the fun function, S is another local object. So the program should print two times ~s () called. (measured vs2015 printed 2 times, only once in GCC and clang)

C++17 Start, S (1) This pure right value is not necessarily materialized to TMP, meaning that in this program, there is no TMP object created at all. Therefore, in a compiler supported by C++17, only one ~s () called is printed.

This is not the compiler's free choice of optimization technology, compiler optimization technology that the S (1) and fun (s) in S is one thing, directly on the structure of s to fuss. (see "Inside C + + Model")

C++17 Mandatory Prvalue is just the initialization semantics, meaning that prvalue can only help other objects to initialize, they can not easily materialized. Here, Fun (S (1)) did not occur temporary materialization. So, when will it happen? For example:

int k = Fun (). m;

In this way, the pure right value returned by the fun () is not materialized, because the object m to access it.

Other physicochemical conditions, see http://en.cppreference.com/w/cpp/language/implicit_conversion#Temporary_materialization

C + + temporary materialization