Concatenate a family of string

Question

I would like to write a function that concatenates any sequence of std::string with just one malloc behind the scene. As a consequence, the total length of the string needs to be computed first. The function needs to be used that way:

std::string s0 = ...;
std::string s1 = ...;
std::string s2 = ...;

std::string s = join(s0, s1, s2);

A better join would use a mix of std::string and std::string_view. It would be even better if we could add string literals. How would you write such a function in C++11 (it needs to compile with gcc 4.8.5 and Visual Studio 2015)?

Answer 1

If you have a string_view type, then you can just accept a collection of string_views, sum their sizes, allocate, then copy in.

std::string join(std::initializer_list<string_view> values)
{
    std::string result;
    result.reserve(std::accumulate(values.begin(), values.end(), 0, [](string_view s) { return s.length(); }));
    std::for_each(values.begin(), values.end(), [&result](string_view s) { result.append(s.data()); });
    return result;
}