C++ Partial Template Specialization

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Core Rules and Constraints

1. Primary Template Requirement: A primary template must be declared before any partial specializations can be defined. The compiler uses the primary template to determine the base number and kind of template parameters.
2. Applicability: Partial specialization applies strictly to class templates and variable templates (introduced in C++14). Function templates cannot be partially specialized; developers must rely on function overloading or if constexpr instead.
3. Parameter vs. Argument Lists: A partial specialization possesses its own template parameter list (template <...>), which dictates the generic types available to the specialization. It also possesses a template argument list appended to the template identifier (ClassName<...>), which dictates the pattern the compiler must match to select this specialization.

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Syntax Visualization

// 1. Primary Template Declaration
template <typename T, typename U>
class Pair {
    // Generic implementation
};

// 2. Partial Specialization: Fixing one type (U is fixed to int)
template <typename T>
class Pair<T, int> {
    // Implementation when the second type is exactly 'int'
};

// 3. Partial Specialization: Modifying a type category (T and U are pointers)
template <typename T, typename U>
class Pair<T*, U*> {
    // Implementation when both types are pointers
};

// 4. Partial Specialization: Constraining types to be identical
template <typename T>
class Pair<T, T> {
    // Implementation when both types are the same
};

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Compiler Resolution Mechanics

1. Pattern Matching: The compiler compares the provided template arguments against the primary template and all available partial specializations.
2. Most Specialized Match: If the arguments match multiple partial specializations, the compiler selects the most specialized template. A template A is considered more specialized than B if A accepts a strict subset of the types that B accepts.
3. Ambiguity Errors: If the compiler finds multiple matches that are equally specialized, it cannot resolve the instantiation and emits an ambiguity compilation error.
4. Fallback: If no partial or full specializations match the provided arguments, the compiler instantiates the primary template.

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Structural Requirements of the Argument List

• Cannot be identical to the primary template: The argument list cannot simply mirror the primary template’s parameters.
• Cannot be fully concrete: If all arguments are concrete types or values, it becomes an explicit (full) specialization, requiring an empty template parameter list (template <>).
• Non-type template parameters: An argument for a non-type template parameter in a partial specialization must be a constant expression. It cannot involve complex expressions relying on the partial specialization’s own template parameters.

template <typename T, int Size>
class Buffer {}; // Primary

// VALID: Partial specialization on the type, leaving the non-type parameter generic
template <int Size>
class Buffer<char, Size> {}; 

// VALID: Partial specialization on the non-type parameter, leaving the type generic
template <typename T>
class Buffer<T, 1024> {}; 

// INVALID: The argument list cannot be an expression of a template parameter
// template <typename T, int Size>
// class Buffer<T, Size * 2> {};