Beginner’s Bug: What’s Wrong with Brace-Init and std::vector?

Question: I have some old C++ code that creates a std::vector containing two NUL wchar_ts:

std::vector<wchar_t> v(2, L'\0');

This code works just fine. I tried to modernize it, using C++11’s brace-initialization syntax:

// Braces {} used instead of parentheses () 
// to initialize v
std::vector<wchar_t> v{2, L'\0'};

However, now I have a bug in my new C++ code! In fact, the brace-initialized vector now contains a wchar_t having code 2, and a NUL. But I wanted two NULs! What’s wrong with my code?

Answer: The problem is that the new C++11 brace initialization syntax (more precisely: direct-list-initialization) in this vector case stomps (prevails) on the initialize-with-N-copies-of-X constructor overload that was used in the original code. So, the vector gets initialized with the content of the braced initialization list: {2, L’\0’}, i.e. a wchar_t having code 2, and a NUL.

I’m sorry: to get the expected original behavior you have to stick with the classical C++98 parentheses initialization syntax.


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