无法推断返回类型的模板参数

我正在尝试使用std:：function 创建一个抽象工厂

一个正常的工厂可以正常工作，但当我想返回一个工厂(另一个std:：函数(而不是一个对象时，我会得到以下错误：

/home/nikolai/Projects/cpplearn/AbstractFactoryPattern/Main.cpp:9:45: error: no matching function for call to ‘factoryProducer(const char [6])’
auto shapeFactory = factoryProducer("shape")();
^
In file included from /home/nikolai/Projects/cpplearn/AbstractFactoryPattern/Main.cpp:3:
/home/nikolai/Projects/cpplearn/AbstractFactoryPattern/AbstractFactory.hpp:48:1: note: candidate: ‘template<class ReturnType> ReturnType abstractfactory::factoryProducer(const string&)’
factoryProducer(const std::string& tag = {})
^~~~~~~~~~~~~~~
/home/nikolai/Projects/cpplearn/AbstractFactoryPattern/AbstractFactory.hpp:48:1: note:   template argument deduction/substitution failed:
/home/nikolai/Projects/cpplearn/AbstractFactoryPattern/Main.cpp:9:45: note:   couldn't deduce template parameter ‘ReturnType’
auto shapeFactory = factoryProducer("shape")();

我对工厂的定义如下：

namespace factory 
{
/**
* A template for a factory, which is just a std::function.
* See AnimalFactory.hpp for example usage.
*/
template <class ReturnType, class ...Args>
using Factory = std::function<ReturnType(Args...)>;
}

工厂生产商工作得很好，除了上一个，我正试图创建一个工厂工厂，退货类型必须是动态的：

namespace abstractfactory 
{
using ShapeFactory = factory::Factory<std::unique_ptr<Shape>>;
using ColorFactory = factory::Factory<std::unique_ptr<Color>>;
ShapeFactory shapeFactoryProducer(const std::string& tag = {})
{
return [=]
{
if(tag == "rectangle")
return std::unique_ptr<Shape>(new Rectangle());
else if(tag == "circle")
return std::unique_ptr<Shape>(new Circle());
else if(tag == "square")
return std::unique_ptr<Shape>(new Square());
else
return std::unique_ptr<Shape>(nullptr);
};
}
ColorFactory colorFactoryProducer(const std::string& tag = {})
{
return [=]
{
if(tag == "green")
return std::unique_ptr<Color>(new Green());
else if(tag == "blue")
return std::unique_ptr<Color>(new Blue());
else if(tag == "red")
return std::unique_ptr<Color>(new Red());
else
return std::unique_ptr<Color>(nullptr);
};
}
template <class ReturnType>
ReturnType
factoryProducer(const std::string& tag = {})
{
return [=]
{
if(tag == "shape")
return shapeFactoryProducer;
else if(tag == "color")
return colorFactoryProducer;
else
return [=] {};
};
}
}

如何实现函数factoryProducer的动态返回类型？

编辑：新代码(仍然不起作用(：

struct FactoryType {};
struct ShapeType {};
struct ColorType {};
template <class TFactory, class ReturnType>
ReturnType factoryProducer(TFactory tag)
{
return [=]
{
if constexpr (std::is_same<TFactory, ShapeType>::value)
return shapeFactoryProducer;
else if constexpr (std::is_same<TFactory, ColorType>::value)
return colorFactoryProducer;
};
}
auto shapeFactoryProducer = factoryProducer(ShapeType());