Attempted construction of temporary object using only class name in declaration

Question

C++

Objects of this class couts messages when they are constructed and destroyed. I tried to construct a temporary object using a declaration of only the class name, but it gave an unexpected output.

In #1, I instantiate a temporary nameless object using parentheses.

In #2, I instantiate a temporary nameless object using uniform initialization.

I didn't know whether #3 would compile. I only thought that if #3 were to compile, it would signify a construction of a temporary nameless object. It does compile, but no object is constructed as seen from the blankness of the console output under #3. What is happening here?

#include <iostream>

class A
{
public:
    A() {std::cout << "constructed\n";}
    ~A() {std::cout << "destroyed\n";}
};

auto main() -> int
{
    std::cout << "#1:\n";
    A();
    std::cout << "#2:\n";
    A{};
    std::cout << "#3:\n";
    A;
    return 0;
}

Console Output:

#1:
constructed
destroyed
#2:
constructed
destroyed
#3:

Note: This was compiled in VC11 with November 2012 CTP. It doesn't compile in g++ 4.8.0 or clang 3.2, which gives error: declaration does not declare anything [-fpermissive] and fatal error: 'iostream' file not found, respectively.

Answer 1

The code is invalid under all C++ standards (C++98, C++03, C++11) and should not compile.

A type is not a statement.

And indeed neither Visual C++ nor g++ compiles it:

Oops, while g++ correctly diagnoses the program, the November 2012 CTP of Visual C++ does not:

[D:\dev\test]
> (cl 2>&1) | find /i "C++"
Microsoft (R) C/C++ Optimizing Compiler Version 17.00.51025 for x86

[D:\dev\test]
> cl /nologo /EHsc /GR /W4 foo.cpp
foo.cpp

[D:\dev\test]
> g++ -std=c++0x -pedantic -Wall foo.cpp
foo.cpp: In function 'int main()':
foo.cpp:17:5: error: declaration does not declare anything [-fpermissive]

[D:\dev\test]
> _

So this is a compiler bug, and you can try to report it at Microsoft Connect.

Answer 2

I tried your code in gcc version 4.5.3, when I compile, it gave the following error message: " error: declaration does not declare anything", what kind of compiler are you using? It is highly possible that you compiler does something under the hood.