C++模板参数推导问题：简化Integrator类实例化

现有C++代码如下：

#include <array>

template <size_t StateSize, typename VariableType>
class DifferentialEquation {
public:
  using StateType = std::array<VariableType, StateSize>;
  using Var = VariableType;
  // ...
};

template <size_t StateSize, typename VariableType>
class RungeKutta4 {
  // ...
};

template<
  template<size_t, class> class IntegratorStrategy,
  class VariableType,
  size_t StateSize
>
class Integrator {
public:
  using DE = DifferentialEquation<StateSize, VariableType>;
  using Strategy = IntegratorStrategy<StateSize, VariableType>;

public:
  Integrator(DE de) : m_de(de) {}
  
  // ...

private:
  DE m_de;
  Strategy m_strategy;
};

其中：

• DifferentialEquation表示微分方程，StateType是std::array类型的状态向量
• RungeKutta4是积分策略类
• Integrator依赖DifferentialEquation完成积分计算

当前可以通过以下代码成功实例化：

DifferentialEquation<2, double> deq;
Integrator<RungeKutta4, double, 2> integrator(deq);

但希望消除模板参数冗余——double和2已经在DifferentialEquation中指定，希望简化为：

DifferentialEquation<2, double> deq;
Integrator<RungeKutta4> integrator(deq);

尝试的推导指南未生效：

template<
  template<size_t, class> class IntegratorStrategy,
  class VariableType,
  size_t StateSize
>
Integrator(DifferentialEquation<StateSize, VariableType>) -> Integrator<
  IntegratorStrategy<StateSize, VariableType>,
  VariableType, StateSize
>;

编译报错（Visual Studio）：

IntegrationTest.cpp(44): error C2955: 'Integrator': use of class template requires template argument list
  IntegrationTest.cpp(23): note: see declaration of 'Integrator'
IntegrationTest.cpp(44): error C3200: 'IntegratorStrategy<StateSize,VariableType>': invalid template argument for template parameter 'IntegratorStrategy', expected a class template
IntegrationTest.cpp(44): error C2955: 'Integrator': use of class template requires template argument list
  IntegrationTest.cpp(23): note: see declaration of 'Integrator'
IntegrationTest.cpp(44): error C2955: 'Integrator': use of class template requires template argument list
  IntegrationTest.cpp(23): note: see declaration of 'Integrator'
IntegrationTest.cpp(44): error C2639: trailing return type 'Integrator' of deduction guide should be a specialization of 'Integrator'
  ninja: build stopped: subcommand failed.

方法1：修正模板推导指南 #

原推导指南的核心错误是：将实例化后的策略类（IntegratorStrategy<StateSize, VariableType>）传给了Integrator的第一个模板参数，但Integrator的第一个参数是模板模板参数，需要接收未实例化的模板（比如RungeKutta4）。

正确的推导指南应直接传递模板模板参数，从构造函数参数中自动推导VariableType和StateSize：

template<
  template<size_t, class> class IntegratorStrategy,
  typename VariableType,
  size_t StateSize
>
Integrator(DifferentialEquation<StateSize, VariableType>) 
-> Integrator<IntegratorStrategy, VariableType, StateSize>;

修改后，Integrator<RungeKutta4> integrator(deq);即可正常编译——编译器会从deq的类型自动推导VariableType=double和StateSize=2，结合指定的IntegratorStrategy=RungeKutta4生成正确实例。

方法2：调整Integrator模板参数顺序（可选优化） #

若想让模板参数逻辑更清晰，可将可推导的参数放在后面：

template<
  template<size_t, class> class IntegratorStrategy,
  size_t StateSize,
  typename VariableType
>
class Integrator {
  // 内部代码保持不变
};

配合修正后的推导指南，实例化代码依然可以正常工作，仅优化了参数顺序的可读性。

验证代码 #

完整可编译示例：

#include <array>

template <size_t StateSize, typename VariableType>
class DifferentialEquation {
public:
  using StateType = std::array<VariableType, StateSize>;
  using Var = VariableType;
};

template <size_t StateSize, typename VariableType>
class RungeKutta4 {};

template<
  template<size_t, class> class IntegratorStrategy,
  class VariableType,
  size_t StateSize
>
class Integrator {
public:
  using DE = DifferentialEquation<StateSize, VariableType>;
  using Strategy = IntegratorStrategy<StateSize, VariableType>;

public:
  Integrator(DE de) : m_de(de) {}

private:
  DE m_de;
  Strategy m_strategy;
};

// 正确的推导指南
template<
  template<size_t, class> class IntegratorStrategy,
  typename VariableType,
  size_t StateSize
>
Integrator(DifferentialEquation<StateSize, VariableType>) 
-> Integrator<IntegratorStrategy, VariableType, StateSize>;

int main() {
  DifferentialEquation<2, double> deq;
  Integrator<RungeKutta4> integrator(deq); // 编译通过
  return 0;
}

内容的提问来源于stack exchange，提问作者Jepessen