Garbage value in implicit conversion from expression `wchar_t + wchar_t[]` to std::wstring

Question

I have come across something weird in Visual Studio C++ 2013 Community Edition which is either a compiler bug or I'm writing invalid code that does compile without warnings.

Consider the following snippet:

#include <string>
#include <iostream>

int main()
{
    std::wstring text;
    wchar_t firstChar = 'A';
    text = firstChar + L"B";
    std::wcout << text << std::endl;

    return 0;
}

I expect the output to be "AB" but what I get is random garbage.

My question: is the operator+ between the wchar_t and wchar_t[] ill-defined, or is this a compiler/library bug? In the case that it is ill-defined, should the compiler have issued a warning?

---

Note: I am not looking for a solution/workaround. Changing L"B" to std::wstring(L"B") fixes the problem. What I want to know is if I did something wrong or if the compiler did.

Note 2: I also get a garbage result in Visual Studio C++ 2010 and an online compiler which supposedly uses g++, and no compilation error, so I'm leaning towards that code being invalid, even though I would expect a compiler error.

Answer 1

firstChar + L"B" increments the address of the string literal L"B" with the promoted value of firstChar. It doesn't concatenate the character and the string. This is undefined behavior since std::basic_string will try to copy the string until it finds L'\0', which is beyond its original bound.

std::basic_string has multiple overloaded operators that allow operations like this to have intuitive behavior, which is why it works in that case.