c++ - How to constrain template methods to match class template parameters? - Stack Overflow

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I have a following class:

template<typename Population, typename... Operators> class GA
{
public:
    template<typename Evaluator,
             typename std::enable_if_t<
             std::is_same_v<Evaluator, Evaluator<Operators...>>>>
  void EvaluatePopulation(Evaluator& evaluator)
  {
    evaluator.Evaluate();
  }
};

This doesn't compile, because of incorrect EvaluatePopulation declaration.

How do I specify that EvaluatePopulation method can be called only with Evaluator class that was initialized with the same Operators... as the GA class?

I have a following class:

template<typename Population, typename... Operators> class GA
{
public:
    template<typename Evaluator,
             typename std::enable_if_t<
             std::is_same_v<Evaluator, Evaluator<Operators...>>>>
  void EvaluatePopulation(Evaluator& evaluator)
  {
    evaluator.Evaluate();
  }
};

This doesn't compile, because of incorrect EvaluatePopulation declaration.

How do I specify that EvaluatePopulation method can be called only with Evaluator class that was initialized with the same Operators... as the GA class?

• c++
• templates
• constraints
• function-templates
• class-template

Share Improve this question Follow edited Mar 30 at 20:17 JeJo 33.5k66 gold badges5353 silver badges9494 bronze badges asked Mar 30 at 17:21 PeterPeter 11111 silver badge88 bronze badges 1

• Can't doublecheck the syntax right now but I think you just need a template-template parameter instead like template<template<typename...> Evaluator> void EvaluatePopulation(Evaluator<Operators...>& evaluator) – NathanOliver Commented Mar 30 at 17:26

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1 Answer 1

Sorted by: Reset to default 9

How do I specify that EvaluatePopulation method can be called only with Evaluator class that was initialised with the same Operators... as the GA class?

The trick is to use a template template parameter so that you “unpack” the evaluator’s template arguments to match the GA’s pack. For example, you can write:

template<typename Population, typename... Operators>
class GA
{
public:
    template<template<typename...> class EvaluatorT>
    void EvaluatePopulation(EvaluatorT<Operators...>& evaluator)
    {
        evaluator.Evaluate();
    }
};

((See live demo))

---

Alternatively, using c++20's concepts, which gives clearer error messages:

// Ensures Evaluator is instantiated with exactly Operators...
template<typename T, typename... Args>
constexpr bool is_evaluator_for_v = false;

template<template<typename...> class EvaluatorT, typename... Ops>
constexpr bool is_evaluator_for_v<EvaluatorT<Ops...>, Ops...> = true;

template<typename Evaluator, typename... Operators>
concept EvaluatorFor = is_evaluator_for_v<Evaluator, Operators...>;

template<typename Population, typename... Operators> class GA 
{
public:
    
    template<typename Evaluator> requires EvaluatorFor<Evaluator, Operators...>
    void EvaluatePopulation(Evaluator& evaluator) 
    {
        evaluator.Evaluate();
    }
};

((See live demo))

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