当该类具有const char *构造函数时，为什么用const char *变量构造该类的未分配临时实例，为什么会出错？

Question

Why does this code (unassigned temporary variable constructed from a const char* variable):

class A
{
public:
    A(const char*) {}
};

int main()
{
    const char* constCharPointerVariable = "StringLiteral";
    A(constCharPointerVariable);
    return 0;
}

Give these errors?

error C2512: 'A' : no appropriate default constructor available
error C2040: 'constCharPointerVariable' : 'A' differs in levels of indirection from 'const char *'

Whereas this code (assigned temporary variable constructed from a const char* variable):

class A
{
public:
    A(const char*) {}
};

int main()
{
    const char* constCharPointerVariable = "StringLiteral";
    A a(constCharPointerVariable);
    return 0;
}

Gives no errors.

And this code (unassigned temporary variable constructed from a const char* variable static_cast to a const char*):

class A
{
public:
    A(const char*) {}
};

int main()
{
    const char* constCharPointerVariable = "StringLiteral";
    A(static_cast<const char*>(constCharPointerVariable));
    return 0;
}

Gives no errors.

Bonus points if you can provide the section number in the C++ specification that specifies the first code sample to be not allowed.

Answer 1

A(constCharPointerVariable);

This is actually a declaration of a variable of type A named constCharPointerVariable . It does not create a temporary object.

If you used clang, you'd get the more helpful error message:

error: redefinition of 'constCharPointerVariable' with a different type
    A(constCharPointerVariable);
      ^

As a simpler example, the following is invalid because it declares two int objects in the same scope, both named x :

int x(0);
int (x);

As for why the code is parsed this way, you can find the syntax rules for Declarators in §A.7 of C++11. Basically, when you declare a variable, you can enclose its name in any number of parentheses.

Relevant productions include:

• declarator -> ptr-declarator
• ptr-declarator -> noptr-declarator | declarator-id
• noptr-declarator -> ( ptr-declarator )