#include <iostream>
#include <string>

struct A
{
    std::string foo() const{ return "aaa"; }
};

void bar(const char* c) {
    std::cout << c;
}

int main()
{
    A a;
    bar(a.foo().c_str()); // Is this safe?
    bar(a.foo().substr().c_str()); // or this
}