User Defined String Literals compared to Const Strings

Question

Consider the following code with these two structures:

std::string operator"" _str(const char* str, std::size_t len) {
    return std::string( str, len );
}

struct MessageLiterals {
    std::string HELP  = "Press F1 for help"_str;
    std::string ABOUT = "Press F2 for about"_str;
    std::string EXIT  = "Press ESC to exit"_str;
};

struct MessageConst {
    const std::string HELP { "Press F1 for help" };
    const std::string ABOUT { "Press F2 for about" };    
    const std::string EXIT { "Press ESC to exit" };
};

int main() {

    MessageLiterals ml;
    std::cout << "Using Literals:\n";
    std::cout << ml.HELP << std::endl;
    std::cout << ml.ABOUT << std::endl;
    std::cout << ml.EXIT << std::endl;
    std::cout << std::endl;

    MessageConst mc;
    std::cout << "Using Constant Strings:\n";
    std::cout << mc.HELP << std::endl;
    std::cout << mc.ABOUT << std::endl;
    std::cout << mc.EXIT << std::endl;

    std::cout << "\nPress any key and enter to quit." << std::endl;
    char c;
    std::cin >> c;

    return 0;
}

A couple of questions come to mind between the two.

1. Are these considered equivalent or not although they produce the same result?
   • Equivalent in the means of "memory foot print", "compile-run time efficiency", etc.
2. What are the pros/cons of each.
3. Are there any advantages - disadvantages one over the other?

I just came across the concept of user-defined literals and I'm trying to get a better understanding of their functionality and usefulness.

EDIT

Okay a little bit of confusion to those trying to answer. I'm familiar with the use of const. The question(s) seems like it is more than one. But generally what I had in mind is harder to put into words or a form of a question, but the overall concept of the difference between the two that I was trying to get at was: Are there any major differences between using "constant std::strings" over "user-defined string literals"?

Answer 1

std::string HELP  = "Press F1 for help"_str;

is no different from

std::string HELP  = "Press F1 for help";

std::string can be constructed from a C-style string.

---

Are these considered equivalent or not although they produce the same result?

Unless the compiler can perform some more aggressive optimization thanks to const, they are the same.

---

What are the pros/cons of each.

const prevents accidental mutation of your string constants.

---

You don't need to use std::string here - you have compile-time constants that could be constexpr:

struct MessageConst {
    static constexpr const char* HELP { "Press F1 for help" };
    static constexpr const char* ABOUT { "Press F2 for about" };    
    static constexpr const char* EXIT { "Press ESC to exit" };
};

The code above guarantees no dynamic allocation and ensures that the constants can be evaluated at compile-time.