Determine the maximal length of the strings falling under the Small String Optimization of your standard library

small_string_optimization_length.cpp

#include <string>
#include <iostream>
#include <cstdlib>

template <typename T = char>
struct failing_alloc : public std::allocator<T> {
    using value_type = T;
    failing_alloc() = default;
    template <class U> failing_alloc(failing_alloc<U> const&) {}
    template <typename U> struct rebind { typedef failing_alloc<U> other; };
    void deallocate(T*, std::size_t) { }
    T* allocate(std::size_t n) {
        std::cout << "Allocating " << n << " bytes\n";
        std::exit(EXIT_FAILURE);
    }

};

template <typename T, typename U>
inline bool operator == (failing_alloc<T> const&, failing_alloc<U> const&) { return true; }

template <typename T, typename U>
inline bool operator != (failing_alloc<T> const& a, failing_alloc<U> const& b) { return !(a == b); }

int main() {
    std::cout << "`sizeof(std::string)` = " << sizeof(std::string) << " bytes\n";

    using stack_str = std::basic_string<char, std::char_traits<char>, failing_alloc<char>> ;
    stack_str str;
    for (std::size_t i = 1; i < 100; ++i) {
        str.push_back('a');
        std::cout << "Appended " << i << " bytes\n";
    }
    return 0;
}