In practice, all modern C++ compilers and standard libraries just set a null character to the c_str()[len] position and reallocate the string if the capacity of

In practice, all modern C++ compilers and standard libraries just set a null character to the c_str()[len] position and reallocate the string if the capacity of the string buffer cannot contain the extra byte. It is never a linear operation unless you're working with old or niche C++ compilers. In C++11 this is required in the standard.