Question

Here is a simple example;

template <typename T>
void foo(T t) {}

std::string str("some huge text");
foo(str);

My question is how can I force the compiler to pass str by reference
without modifying function foo?

Answer 1

Pass the reference type explicitly:

template <typename T>
void foo(T t) {}

int main() {
   std::string str("some huge text");
   foo<std::string&>(str);
}

This does modify the function instantiation that you get (by generating a void foo<std::string&>(std::string& t)), but it doesn’t modify the function template.

Live demo.