#include <type_traits>

template <typename T>
inline constexpr std::remove_reference_t<T> &lvalue(T &&r) noexcept {
    return static_cast<std::remove_reference_t<T> &>(r);
}

std::string get_my_file() {
    auto ifs = std::ifstream("myfile.txt");
    return {std::istreambuf_iterator<char>(ifs),
            std::istreambuf_iterator<char>()};
}

std::string get_my_file() {
    return {std::istreambuf_iterator<char>(lvalue(std::ifstream("myfile.txt"))),
            std::istreambuf_iterator<char>()};
}

std::string temp1 = get_my_shader();
const char *temp2 = temp1.c_str();
glShaderSource(a, 1, &temp2, nullptr);

glShaderSource(a, 1, &lvalue(get_my_shader().c_str()), nullptr);