In C++ templates, what is meant by template specialisation?

Introduction / Context:
Template specialisation is an important advanced feature of C++ templates. It allows library authors to provide special behaviour for particular template arguments while still offering a generic implementation for other cases. This question asks you to describe what template specialisation means in this context.

Given Data / Assumptions:

• We are using C++ templates for classes or functions.
• There is a primary template that defines generic behaviour for a range of types or values.
• We want to adjust that behaviour when certain template arguments are used.

Concept / Approach:
Template specialisation lets you write a specific version of a template for a given set of template arguments. In full specialisation, you provide a complete alternative definition when all template parameters match certain values or types. In partial specialisation, you specialise a subset of parameters, such as all pointer types or all containers of a particular form. When the compiler encounters a use of the template, it chooses the most specialised matching definition. This enables optimised or tailored behaviour for special cases without losing the convenience of a general template for the rest.

Step-by-Step Solution:
Step 1: Recall that primary templates provide default generic behaviour.Step 2: Recognise that specialisation occurs when you override this behaviour for a specific set of template parameters.Step 3: Option A states that template specialisation means providing a custom implementation for a particular data type or set of arguments.Step 4: Options B, C and D describe restrictions or deprecations that are not the purpose of specialisation.Step 5: Option E talks about where templates are placed in files, which is unrelated to the concept of template specialisation, so option A is correct.

Verification / Alternative check:
Consider a simple primary template template<typename T> struct Traits { static const bool is_pointer = false; };. You can then define a specialisation template<typename T> struct Traits<T*> { static const bool is_pointer = true; };. When you use Traits, the primary template applies, and is_pointer is false. When you use Traits, the specialised version applies and is_pointer is true. This is a classic example of template specialisation in action.

Why Other Options Are Wrong:
Option B suggests that specialisation is about default arguments, which is not accurate. Option C describes deprecation, which is controlled by attributes or compiler warnings, not by specialisation. Option D implies that specialisation is only about pointer types, but pointers are just one use case. Option E talks about the placement of code in header or source files, which is a separate issue related to template instantiation rules.

Common Pitfalls:
Developers sometimes overuse specialisation, leading to complex and hard to maintain code. Another pitfall is confusing full specialisation with partial specialisation and expecting both to work identically for function templates. Function templates do not support partial specialisation in the same way class templates do. In exam settings, focus on the core idea that template specialisation provides custom implementations for particular template arguments.

Final Answer:
Providing a custom implementation of a template for a specific data type or set of template arguments instead of using the primary generic definition.