Chaos in C ++: The mysteries of template parameters

Source: Internet
Author: User



Let's start with a question

Template<TypenameT, T * P>




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.


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


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.

// The End

Related Article

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: and provide relevant evidence. A staff member will contact you within 5 working days.

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

  • Sales Support

    1 on 1 presale consultation

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.