Question

template <bool, class t, class u>// why is bool here,class booltype=bool. Are they equivalent?
struct if_
{
    typedef typename t type;
};

template<class t, class u>
struct if_<false,  t,  u>   // what does the <false,t,u> mean？   
{
    typedef typename u type;
};

The code if from an article named “name the template parameter “. I can’t understand the definition of the both struct.

Answer 1

EDIT: Moved the most important part first:

if_<true, int, double>::type;  // this is int
if_<false, int, double>::type; // this is double

This conveniently defines a type that can conditionally be defined at compile-time.

Old Answer:

template <bool, class t, class u>// why is bool here,class booltype=bool. Are they equivalent?
struct if_
{
    typedef typename t type;
};

This means the first parameter passed to the template is a bool.

It’s not equivalent to class booltype=bool. That would be a default typename for the template.

template<class t, class u>
struct if_<false,  t,  u>   // what does the <false,t,u> mean？   
{
    typedef typename u type;
};

This is a specialization of your struct. If the first parameter passed to the template is false, this it what the struct is defined as.

Basically, assume:

if_<true, int, double> x;
//is defined as
//struct if_
//{
//    typedef int type;
//};

and

if_<false, int, double> x;
//is defined as
//struct if_
//{
//    typedef double type;
//};

The notation basically tells what to typedef – if the first parameter is true, typedef the second parameter, otherwise the third.