Chaos in C ++: The mysteries of template parameters

Difficulty:

 

Let's start with a question

Template<TypenameT, T * P>

StructA

{};

 

If there is an int type object I, then for the following code

A <Int, & I> OBJ;

Is this legal?

 

 

A: A <Int, & I> OBJ; may be legal or may be invalid.

What can be used as a template parameter? Some built-in types and user types, and some non-types can also be used as template parameters.

One requirement of non-type template parameters is that the compiler can determine the parameters during compilation. In other words, a non-type template parameter must be a compilation constant.

To determine whether this sentence is valid, check whether the returned value of & I is a compilation constant. When I is a global or static object, this statement is correct, because the memory allocation of global and static objects occurs during the compilation period, so the address of I (& I value) it can be determined.

Now it is legal to complete this program.

IntI;

IntMain (){

A <int, & I> OBJ;

}

If the second parameter of this template is a reference, the same is true. However, it is worth noting that these non-type and non-reference template parameters are not left values!

Finally, there are integers, Enum types, pointers, and references that can be used as non-type parameters.

 

The local user-defined type (Local class) Cannot be used as template parameters. This is because local classes do not have external connections. For example

Template<TypenameT>

ClassTest {};

 

VoidFun1 ()

{

StructX {};

Test <x>;

}

 

VoidFun2 ()

{

StructX {};

Test <x>;

}

The above test <x> A; is it the same thing? Because there is no external connection, they are the same thing. The programmer's intention is that two local Classes X are two different class definitions, and test <A> is considered to be two different template instances.

For a local class, it can be said that it is an abnormal form of a sound C ++ system. Without an external connection, it cannot own static data members or template member functions.

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