为什么char {}和char（）作为char *参数的临时变量？

Question

In Visual C++ 2017 (with /std:c++14 or with /std:c++17 ), the following code works:

void TakePtr(char*); // const or not

int main()
{ 
     TakePtr(char{});
     TakePtr(char());
}

I don't understand why it works.

Apparently, the following would also work (as expected):

void TakeChar(char);

   TakeChar(char{});
   TakeChar(char());

How does the compiler deduce (or convert) the type char to char* , when char{} or char() is used as an argument?

Now, if I have both char and char* overloads, it works without any error/warning about ambiguity:

void TakePtr(char*);
void TakePtr(char);

    TakePtr(char{});  // Chooses 'char'
    TakePtr(char());  // Chooses 'char'

Why is the compiler okay with char{} for TakePtr(char*) ? And why doesn't it give a warning/error when choosing the better version? Such behavior is bound to break existing code.

For sure, the compiler isn't happy with:

void TakePtr(char*);

    char c{};
    TakePtr(c);

Answer 1

Because Visual lies a lot. Especially older one. Your code prompts clang to report an error:

<source>:9:6: error: no matching function for call to 'TakePtr'

     TakePtr(char{});

     ^~~~~~~

<source>:5:6: note: candidate function not viable: no known conversion from 'char' to 'char *' for 1st argument

void TakePtr(char*); // const or not

     ^

<source>:10:6: error: no matching function for call to 'TakePtr'

     TakePtr(char());

     ^~~~~~~

<source>:5:6: note: candidate function not viable: no known conversion from 'char' to 'char *' for 1st argument

void TakePtr(char*); // const or not

     ^

2 errors generated.

Visual is known to be "wonky" in term of following C++ standard, so don't rely on it too much. Try to verify with clang / gcc, just to be sure.

Answer 2

This is simply MSVC being behind: the rule in C++03 was that any constant expression of integer type and value 0 was a null pointer constant and could thus be converted to char* . Certainly char() qualifies—and char{} means the same thing, although it never overlapped with the rule.