Question

Please, explain, why in the following code there is no error:

void f()
{
}

void h()
{
    std::bind(f)(42);
}

Why compiler doesn’t complain about redundant parameter in std::bind while binding function f? And if it shouldn’t, please explain why this can be useful.

Answer 1

This code is conformant. The arguments that you pass to the result of bind are only used if needed.

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Using the terminology from the standard: u is the result of std::bind(f, t1, ..., tN).

Approximately speaking:

When u(u1, u2, ..., uM) is called, f is called as f(v1, ..., vN), where the values of vi are determined by the following algorithm:

//N is the N from `std::bind(f, t1, ..., tN)`
For each i in 1 to N:
    if (ti is a reference wrapper) vi is the unwrapped version of t1
    if (ti is a bind_expression) vi is the result of calling ti with u1, ..., uM
    if (ti is a placeholder) vi is uj (where j is is_placeholder<decltype(ti)>::value)
    otherwise vi is ti