std::ref using with template function and function passing by value

Question

I have two functions:

void fun(int k) {
    ++k;
}

template<class T> 
void inc(T t) {
    ++t;
}

And I call these functions with std::ref:

int p = 5;
int o = 5;
fun(std::ref(p));
inc(std::ref(o));

After that value of p is 5 and value of o is 6. What is the difference when I call function with std::ref for these functions?

Answer 1

std::ref returns an std::reference_wrapper, which is implicitly convertible to the type it's wrapping.

When calling fun(std::ref(p)), you're implicitly converting the newly crated std::reference_wrapper to int, making a copy.

When calling inc(std::ref(p)), you're copying the std::reference_wrapper itself, preserving reference semantics.