move constructor is not getting called when I am calling constructor explicitly as a parameter

Question

I am trying to learn about move constructors. I wrote the below program.

#include <iostream> #include <algorithm> using namespace std; class myclass { public: myclass() { cout << "In Constructor" << endl; } ~myclass() { cout << "In Destructor" << endl; } myclass(const myclass &obj) { cout << "In Copy Constructor" << endl; } myclass(myclass &&obj) { cout << "In Move Constructor" << endl; } }; int main() { myclass obj = myclass(); // Line 1 myclass obj(myclass()); // Line 2 }

Line 1 is working as expected, move constructor is getting called if I use '-fno-elide-constructors' flag. But for line 2, nothing is happening. Not even constructor is getting called. I thought that move constructor will be called for line 2 also. But no function is getting called. I know if I call the constructor explicitly, the object will be destroyed at the end of the expression. But I am not sure why even constructor is not getting called. Can any one please let me know what is wrong with Line 2?

Brian Bi · Accepted Answer · 2019-08-27 22:21:58Z

This is an example of the most vexing parse; obj is declared to be a function, rather than an object of type myclass. Most instances of the most vexing parse can be avoided by changing one (or both) pairs of parentheses to braces. For example:

myclass obj(myclass{});

Note that even in C++11, the compiler may optimize out the move (with both direct-initialization and copy-initialization). In C++17, the move is guaranteed to not occur; the object will simply be value-initialized.

FYI: please enable the compiler warning for this! (And promote it to error), you really never need it to become function declaration

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