Is this correct behavior of template?

Question

template<class blah, class bleh>
blah func(bleh p)
{
    // Do something
}

int main()
{
    double d=1.111;
    int i = func<int>(d); // #1
    int j = func<int,double>(d); // #2
    // ....
}

In this example both the instances of func, #1 and #2 are compiling, but I'm unsure of what is correct, and why.

Can some one explain why #1 is correct, and maybe give some background?

Answer 1

Yes, this is correct behaviour.

Case 1 - type deduction

func<int>(d);

This uses template type deduction to determine the type for bleh.

In order to instantiate a function template, every template argument must be known, but not every template argument has to be specified. When possible, the compiler will deduce the missing template arguments from the function arguments. This occurs when a function call is attempted and when an address of a function template is taken.

The compiler sees the type for d as being a double and thus deduces the actual type for bleh must also be a double.

From cppreference, also covered in § 14.8.2 of the C++ specification;

Template argument deduction attempts to determine template arguments ..., which can be substituted into each parameter P to produce the type deduced A, which is the same as the type of the argument A, ... .

If there are multiple parameters, each P/A pair is deduced separately and the deduced template arguments are then combined. If deduction fails or is ambiguous for any P/A pair or if different pairs yield different deduced template arguments, or if any template argument remains neither deduced nor explicitly specified, compilation fails.

Case 2

func<int,double>(d);

The type for bleh is explicitly set to double, hence the compiler will make it such. The argument d is provided and since it is also a double, the compiler happily continues. If an argument (i.e. in place of d) was provided with a type that was not a double, or could not implicitly be converted to a double (e.g. via promotions, non-explicit constructors or user provided conversions), this would result in an error.