what's wrong with this anonymous constructor call?

Question

The following code compiles fine under GCC and MSVC:

#include <string>

class C
{
public:
   C(const std::string&) { }
};

void f()
{
    std::string s;
    const std::string& sr = s;
    C c = C(sr);
}

Furthermore, if I replace that (entire) last line with this, both compilers accept it:

(C(sr));

However, if I replace that last line with this:

C(sr);

Compilers report the following:

GCC:

$ c++ --std=c++17 yo.cpp
yo.cpp: In function ‘void f()’:
yo.cpp:13:9: error: conflicting declaration ‘C sr’
C(sr);
^
yo.cpp:12:24: note: previous declaration as ‘const string& sr’
const std::string& sr = s;

MSVC:

cl /std:c++17 Source.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.16.27043 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
Source.cpp
C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Tools\MSVC\14.16.27023\include\xlocale(319): warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
Source.cpp(14): error C2040: 'sr': 'C' differs in levels of indirection from 'const std::string &'
Source.cpp(14): error C2512: 'C': no appropriate default constructor available
Source.cpp(3): note: see declaration of 'C'

Answer 1

This statement:

C(sr);

is not calling a constructor. The syntax is actually a declaration of a variable named sr of type C, with additional redundant parentheses around the declarator sr.

Since sr has already been declared, you get an error.

On the other hand, in this statement:

(C(sr));

the expression C(sr) is actually parsed as the initialization of a C object, where sr is passed as an argument. The temporary C object that gets created is then destroyed at the end of the full expression.