Class template function instantiation

You must instantiate the template function by using the specified explicit template parameters in at least one linked module if the function is called, or you will get a linker error when the program is built.

The extern keyword in the specialization only applies to member functions defined outside of the body of the class. Functions defined inside the class declaration are considered inline functions and are always instantiated. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. The compiler will automagically select the correct specialization when it sees which T you are using.

One of several ways I personally use template specialization is for stringification. I usually use a template to stringify various objects of various types , but I often need to specialize the code for stringifying certain specific types. For instance, when stringifying bool s I prefer "true" and "false" over "1" and "0" so I use std::boolalpha when T is bool. Also I often prefer floating point output to contain all the digits so I can see very small differences, etc.

The end result usually looks something like this:. Conceptually they all do the same thing: stringify the parameter. That means the observable behavior is consistent, therefore the specializations do not confuse callers. However the details for implementing that observable behavior is slightly different for bool and floating point types, so template specialization is a good approach.

If you blindly applied the advice from the FAQ on template specialization , you would end up duplicating all that code before and after the pseudo-switch statement.

The main foo function would be a simple template — no specializations. It all gets figured out automatically. This is a quality-of-implementation issue so your results may vary. However there is usually no slow-down at all. If anything, the templates might affect the speed of compilation slightly, but once the types are resolved by the compiler at compile-time, it will typically generate code that is just as fast as with non-template functions, including inline-expanding appropriate functions, etc.

Function templates participate in name resolution for overloaded functions, but the rules are different. For a template to be considered in overload resolution, the type has to match exactly.

If the types do not match exactly, the conversions are not considered and the template is simply dropped from the set of viable functions. If all you want to know is how to fix this situation, read the next two FAQs. But in order to understand why things are the way they are, first accept these facts:. Suppose you have a template Foo defined like this:. Now suppose you have some code in file Bar. But if you had put the definition of the constructor and someMethod into file Foo.

A note to the experts: I have obviously made several simplifications above. If you know the difference between a. Reminder: Read the next two FAQs for some solutions to this problem. Watch this space for updates in the near future!! As an example, consider the header file foo. Function templates can be explicitly instantiated by declaring the template with a particular type as an argument.

For example, the following code is allowed:. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Class template Function template. Inline assembly. Compiler support. Freestanding and hosted. Language support library. Technical specifications. Flow control.

Function declaration. Lambda function expression. Fundamental types. Function types. Compound types. Storage duration specifiers. Default initialization. Value initialization. Zero initialization. Copy initialization. Direct initialization. Aggregate initialization. Constant initialization.