I am trying to solve a problem where I can define a class given an std::string only once, or else the compiler will throw an error.
Example:
Suppose we have a class Car which is initialized with its license plate. When creating it, the compiler is supposed to check if a Car instance with the exact same license plate string has already been created before.
My approaches so far:
I have been looking around for template-metaprogramming solutions which should create a simple registerable counter (example), however I fear that this doesn't really fit my needs.
Another idea was to create a simple define including the given license plate string, which but of course are unable to be created at compile-time since the license plate string is being passed only at run-time.
#include <string>
#include <vector>
#include <algorithm>
#include <iostream>
std::vector<std::string> myGlobalVector {};
class Car {
public:
Car(std::string const& licensePlate);
~Car() = default;
// ...
private:
std::string _plate;
};
// ...
Car::Car(std::string const& licensePlate)
{
// this would be the runtime version of what I want to achieve:
const bool alreadyExists = std::any_of(
myGlobalVector.begin(),
myGlobalVector.end(),
[&licensePlate](std::string const& otherPlate)
{
return otherPlate == licensePlate;
});
if (alreadyExists)
{
std::cerr << "License plate already registered. Exiting." << std::endl;
exit(-1);
}
myGlobalVector.emplace(licensePlate);
}
int main() {
Car someCar { "A4EM21F" };
Car anotherCar { "F121EG4" };
// ...
// this should throw a compile-time error as
// given string has already been used before in this context
Car lastCar { "A4EM21F" };
}
My only idea so far (which obviously won't compile but is supposed to illustrate what I want to achieve):
// ...
Car::Car(std::string const& licensePlate)
{
#ifndef CAR_##licensePlate
#define CAR_##licensePlate
_plate = licensePlate;
#else
#error Car has already been created in your code!
#endif
}
// ...
Can anyone think of a way to check the occurrence of a string in a code snippet at compile-time?
I woult be grateful for helpful hints using any kinds of template-metaprogramming, type-trait or other topics.
