Scope of C++ references and STL containers

Question

#include <iostream>
#include <string>
#include <vector>

class SampleClass
{
    public:
        SampleClass(){}
        
        void ReceiveNewMessage()
        {
            const auto msg = GenerateSomeRandomMessage();
            StoreUniqueMessage(msg);
        }
        
    private:
        void StoreUniqueMessage(std::string msg)
        {
            messages.push_back(msg);
        }
    
        std::string GenerateSomeRandomMessage()
        {
            return "Random message";
        }
        
        std::vector<std::string> messages{};
    
};

int main() {
    
    SampleClass sc;
    
    sc.ReceiveNewMessage();

    return 0;
}

Clang is suggesting to use the following signature:

void StoreUniqueMessage(const std::string& msg);

However, as references are an alias for its referent, the behaviour is undefined when the referent is destroyed (correct me if wrong); in this case, within ReceiveNewMessage(), the variable msg is local to this function, but, if I use a reference to it in StoreUniqueMessage(), wouldn't it be undefined behaviour when the function is out of scope? What am I missing about references?

Answer 1

If you have const std::string &msg and a std::vector<std::string> messages, the operation messages.push_back(msg); makes a copy.

The vector owns its contents and takes care of them.

That the reference becomes invalid after that is no longer a problem.

If you had a vector of pointers to strings that someone else owns, the situation would be different.