Template Metaprogramming

Question 1

What are template functions?

Answer 1

When we write the templates of functions that don’t necessarily explicitly specify the types of parameters/return types for function.

Question 2

What does it mean that template functions are candidate functions?

Answer 2

When you attempt to use a function, the compiler will search for all templated functions and try to find a suitable candidate.

Question 3

What happens if a compiler find a suitable templated function?

Answer 3

It will create an instance of that function for the specified types.

Question 4

What is each instance of a template function in the binary?

Answer 4

Each instance of the templated function is a different function in the binary.

Question 5

Where do we write template functions?

Answer 5

Only in the header files. As they are not implementations.

Question 6

How do constrain templates?

Answer 6

Using them in a way that defines their behaviour. E.g. using them in a to string function means type must support &laquo_space;operator

Question 7

Why might template functions printing a class not work?

Answer 7

Because the print function of a class might not be defined.

Question 8

What happens to function calls during linking in compilation?

Answer 8

During compilation, when the compiler comes across a function call it must resolve that function call to a specific instance of a function.

Question 9

During linking what happens if there is a perfectly matching implemented function?

Answer 9

It will link the call to the function implementation.

Question 10

During linking what happens if there is not a perfectly matching implemented function?

Answer 10

• The compiler will look through all the template functions

Question 11

What happens in you include a template in an implementation file that wasn’t used?

Answer 11

The template would not be included in an object file. (Why it needs to be in a header file)

Question 12

What happens during linking in template metaprogramming?

Answer 12

During compilation, if the compiler reaches a call to a function that has no implementation, rather than throw and error and abort compilation, it will try to find a candidate function to create a specialisation of it.

Question 13

What is the compiler process for finding candidate function?

Answer 13

• Compiler creates a set of all functions with a matching name
• Then tries to substitute in the type(s) from the call into the implementation
• If one matches, the compiler will create an instance of the specialisation of the function for a given type
• If one doesn’t match, it will move onto the next template in the set
• An error only unfolds if there are no possible candidate template functions
  (Process also known as SFINAE)

Question 14

What does SFINAE stand for?

Answer 14

Substitution Failure Is Not An Error

Question 15

What is SFINAE?

Answer 15

The concept of metaprogramming where the compiler tries to find a candidate function.

Question 16

What are template classes?

Answer 16

Classes that are defined using templates, allowing them to parameterised with one more types or non-type parameters.

Question 17

What can templated functions and classes be proposed to the compiler for?

Answer 17

Template functions and classes are provided to the compiler as blueprints and the compiler decides whether to include them in the final executable program based on whether they are actually used in the codebase.

Question 18

What does writing our own specialisations allows us to do?

Answer 18

Provide a different implementation for specific substitutes of the template parameters.

Question 19

What is specified in full specialisation?

Answer 19

We provide a type for all template parameters in the original template

Question 20

What is partial specialisation?

Answer 20

(Classes only) we provide types for some but not all template parameters.

Question 21

Why might template metaprogramming be bad for large projects?

Answer 21

As it reconciles the code each time, it can add a lot of time to computation.

Question 22

What can we do to work around the compiler going through all possible templates?

Answer 22

Specify a base type or class as a template parameter.

Question 23

What happens if templates are never called?

Answer 23

If templates are never called, nothing will be included in the binary

Question 24

Can template functions be partially specialized?

Answer 24

No

Question 25

What type of specialization can template functions be?

Answer 25

Only fully specialized.

Question 26

What type of specialization can template classes be?

Answer 26

Fully or partially specialized.

Question 27

What are template specializations?

Answer 27

Examples of template functions with certain types, similar to polymorphism how a function or class can have the same function with different parameters and arguments.

Question 28

What is full specialization?

Answer 28

All template parameters have been specified