struct Base
{
    virtual void foo() const { std::cout << "base\n"; }
};

struct Derived: Base {
    virtual void foo() { std::cout << "derived\n"; }
    private: std::string trash {"123456789123456789"};
};

int main() {
    std::vector<Base> vec;
    vec.push_back(Derived{});
    for(const auto el: vec) {
        el.foo();
    }
}

sum(std::string{""}, "abc");
sum(0, 1.0);

SP>sum(std::string{""}, "abc");
SP>sum(0, 1.0);
SP>

#include <iostream>

template <typename T, typename...X>
decltype(auto) sum(T&& t, X&&...x)
requires requires {(std::forward<T>(t) + ... + std::forward<X>(x));}
{
    return (std::forward<T>(t) + ... + std::forward<X>(x));
}

int main() {
    std::cout << sum("Hello", std::string(", World"), '!') << std::endl;
}

#include <iostream>

auto sum(const auto& t, const auto&...x)
requires requires {(t + ... + x);}
{
    return (t + ... + x);
}

int main() {
    std::cout << sum("Hello", std::string(", World"), '!') << std::endl;
}

R>Так норм?

template<typename T1, typename T2>
auto sum(T1 v1, T2 v2) {
    return v1 + v2;
}

template<typename T>
T sum(T v1, T v2) {
    return v1 + v2;
}

#define SUM(a,b) a+b

_>>>#define SUM(a,b) a+b

#include <iostream>

#define SUM(a,b) a+b

int main() {
    std::cout << SUM(1<<1, 1) << std::endl;
}

_>>#define SUM(a,b) a+b
_>>

_>>

#define SUM(a,b) (a)+(b)