Abbreviated Function Template

An abbreviated function template is a C++20 feature that allows the declaration of function templates using the auto placeholder type specifier in the function’s parameter list, bypassing the need for an explicit template <typename T> declaration. When the compiler encounters auto in a parameter list, it implicitly generates a standard template type parameter for that function.

Syntax and Compiler Translation

The abbreviated syntax acts as direct syntactic sugar for traditional template declarations. Each instance of auto in the parameter list introduces a distinct, independent template parameter.

Single Parameter:

// Abbreviated syntax
void process(auto x);

// Equivalent compiler translation
template <typename T>
void process(T x);

Multiple Parameters: Every auto generates a unique type parameter. They are not implicitly forced to be the same type.

// Abbreviated syntax
void compare(auto a, auto b);

// Equivalent compiler translation
template <typename T1, typename T2>
void compare(T1 a, T2 b);

Variadic Abbreviated Function Templates

Abbreviated syntax fully supports variadic templates. Using auto... implicitly generates a template parameter pack, allowing the function to accept an arbitrary number of arguments of varying types.

// Abbreviated variadic syntax
void print(auto... args);

// Equivalent compiler translation
template <typename... Ts>
void print(Ts... args);

Constrained Abbreviated Templates

Abbreviated function templates integrate directly with C++20 Concepts. You can constrain the implicit template parameter by prepending a concept name to the auto keyword.

#include <concepts>

// Abbreviated syntax with a concept constraint
void calculate(std::integral auto x, std::floating_point auto y);

// Equivalent compiler translation
template <std::integral T1, std::floating_point T2>
void calculate(T1 x, T2 y);

Reference and CV-Qualifiers

The auto keyword in this context obeys standard template type deduction rules, including the application of const, volatile, and reference modifiers.

Notably, using auto&& creates a forwarding reference (often called a universal reference), identical to T&& in a traditional template.

// Abbreviated syntax with qualifiers
void mutate(const auto& read_only_val, auto&& forwarding_ref);

// Equivalent compiler translation
template <typename T1, typename T2>
void mutate(const T1& read_only_val, T2&& forwarding_ref);

Mixing Explicit and Implicit Template Parameters

You can mix traditional explicit template parameter lists with abbreviated auto parameters. When mixed, the implicit template parameters generated by auto are appended to the end of the explicit template parameter list in the order they appear left-to-right.

// Mixed syntax
template <typename T>
void transform(T target, auto modifier);

// Equivalent compiler translation
template <typename T, typename U>
void transform(T target, U modifier);

Technical Considerations

1. Type Enforcement and Scope: Every auto introduces a distinct type parameter. Attempting to enforce type equality inline using decltype (e.g., void func(auto a, std::same_as<decltype(a)> auto b);) results in a compilation error. Concept constraints applied to auto parameters are lifted to the implicitly generated template parameter list, which is evaluated before the function parameters are in scope. To enforce type matching while retaining abbreviated syntax, a trailing requires clause must be used, as function parameters are in scope at that point:

#include <concepts>

// Correct way to enforce type matching with abbreviated syntax
void func(auto a, auto b) requires std::same_as<decltype(a), decltype(b)>;

Alternatively, traditional template syntax (template <typename T> void func(T a, T b);) is generally the preferred approach for enforcing identical types across multiple parameters.

2. Explicit Template Arguments: Invoking an abbreviated function template with explicit template arguments (e.g., process<int, double>(x, y)) is highly brittle. If the function signature is later modified—such as changing an auto parameter to a concrete type—it silently alters the arity and indexing of the implicit template parameter list. This shifts the indices of the remaining template parameters, which will break existing explicit template instantiations at the call site.

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