Assigning a number to std::string is now allowed?

Question

Consider the following code snippet:

#include <iostream>
int main() {
    std::string foo;
    foo = -1; // why is the compiler not complaining about this?
    std::cout << "1" << std::endl;
    std::cout << foo << std::endl;
    std::cout << "2" << std::endl;
}

Actual output (both ideone.com C++14 mode and GCC 4.8.4):

<no output>

Questions:

1. Why did the code snippet compile at all?
2. Commenting out foo = -1, I get the correct stdout (1 and 2). What has the compiler compiled with foo = -1; that causes the subsequent couts to fail?

Answer 1

foo = -1;

resolves to std::string::operator=(char) since -1 is an int and int can, in theory, be converted to a char.

It's not clear to me what the standard says when the int does not represent a valid char. It looks like in your implementation, the program crashes.

Update

From the C++11 Standard (emphasis mine):

3.9.1 Fundamental types

1 Objects declared as characters (char) shall be large enough to store any member of the implementation’s basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values.

It appears that you'll have to consult your compiler's documentation to understand whether it allows char object to hold negative values and, if it does, how does it treat such objects.

Answer 2

char is an integral type in C++. std::string defines an assignment operator:

std::string& operator=(char);

Since int converts to char freely in this context, no diagnostic is given. (It's funny how best intentions pave the road to Hell, innit?)

Since (char)-1 is probably not a valid member if the execution character set on your platform, the stream enters an error state and will stay there, outputting nothing, until the error bit is cleared.

EDIT this is a bug of ideone. If the output stream contains an "illegal" character, the entire stream is not shown, even the parts produced and flushed before the bad character. Use another online compiler to check.