In constexpr function breakpoint is hit

Question

Here is my code:

#include <string.h>
#include <stdlib.h>


template <int ...I>
class MetaString
{
    char buffer_[sizeof...(I)+1];
public:
    // A constexpr constructor 
    constexpr MetaString(const char * arg) :buffer_{ encrypt(arg[I])... }
    {}
    constexpr const char *get()const { return buffer_; }
private:

    constexpr char encrypt(const char c) const { return c ^ 0x55; }
};

char *decrypt(const char* buffer_, int size)
{
    char* tmp = (char *)malloc(size + 1);

    strcpy_s(tmp, size + 10, buffer_);

    for (int i = 0; i < size; i++)
    {
        *(tmp + i) = *(tmp + i) ^ 0x55;
    }

    return tmp;
}

int main()
{
    constexpr MetaString<0,1,2,3,5> var("Post Malone");

    char * var1 = decrypt(var.get(), 5);
    std::cout <<  var1 << std::endl;

    return 1;
}

The idea is simple, I create object of MetaString and provide some string to it. The constructor encrypts the argument by XOR. Then I have decrypt function which decrypts value back.

The problem is that I set breakpoint in constructor (specifically this line constexpr MetaString(const char * arg) :buffer_{ encrypt(arg[I])... }) and it is hit when I run in debugging mode. Which as I understand means that the constructor is called during runtime.

To guarantee that functions be evaluated at compile time I created object this way constexpr MetaString<0,1,2,3,5> var("Post Malone"); But I've read that constexpr variable must be literal type.

So my question is how can I manage to have variable like var (which would have encrypted data in it and be evaluated at compilation time) and then call decrypt at runtime and get original value?

Answer 1

constexpr only guarantees that a function or variable can be used in a constant expression. It does not guarantee that a function/object is going to always be evaluated/constructed at compiletime. In your particular case, that's actually not really possible since we're talking about an object with automatic storage duration. The object, if it is going to be created, can only really be created when the program is running. Try making your variable static…