SFINAE用于检测非成员模板功能的存在

Question

TL; DR我想写一个模板函数Process(T value),根据非成员函数的存在,它对不同的值表现不同CreateProcessor<T>().我能做些什么？

我有SFINAE的问题.假设我们需要支持CreateProcessor返回IProcessor<T>某种类型类型的接口实现的函数T.

在C++中,我们不能创建仅在返回类型上有所不同的函数的多个重载,因此我们必须使函数CreateProcessor也是由参数化的模板函数T.

现在假设我们要编写一个模板函数Process<T>(T value),它根据存在的不同而工作CreateProcessor<T>(),即它应该value在CreateProcessor<T>()实现的情况下使用处理器进行处理,否则会导致错误.

我试图编写以下代码:

#include <cstdio>
#include <type_traits>

// A workaround for void_t as described here: http://en.cppreference.com/w/cpp/types/void_t.
template<typename... Ts> struct make_void { typedef void type;};
template<typename... Ts> using void_t = typename make_void<Ts...>::type;

// An interface for a processor that receives a value of specific type.
template<class T>
class IProcessor {
public:
    virtual void process(T value) = 0;
};

// A processor for int.
class IntProcessor : public IProcessor<int> {
public:
    virtual void process(int value) override {
        printf("IntProcessor::process is called for value = %d\n", value);
    }
};

// Template prototype.
template<class T> 
IProcessor<T>* CreateProcessor();

// Template specialization for int.
template<>
IProcessor<int>* CreateProcessor() {
    return new IntProcessor();
}

// Detector of CreateProcessor.
template<class, class=void>
struct CreateProcessorImplemented : std::false_type { };

template<class T>
struct CreateProcessorImplemented<T, void_t<decltype(CreateProcessor<T>())>> : std::true_type { };


// Specializations depending on existence of CreateProcessor.
template <typename T>
typename std::enable_if<CreateProcessorImplemented<T>::value, void>::type Process(T value) {
    IProcessor<T>* processor = CreateProcessor<T>();
    processor->process(value);
}

template <typename T>
typename std::enable_if<!CreateProcessorImplemented<T>::value, void>::type Process(T value) {
    printf("Processor for requested typename is unavailable\n");
}


int main() {
    Process(42);
    Process("abc");

// static_assert(!CreateProcessorImplemented<char const*>::value, ":(");
/* This static_assert fails with an error:
 * code.cpp:56:5: error: static assertion failed: :(
 *      static_assert(!CreateProcessorImplemented<char const*>::value, ":(");
 */
}

虽然这会导致链接错误:

/tmp/ccTQRc9N.o:code.cpp:function std::enable_if<CreateProcessorImplemented<char const*, void>::value, void>::type Process<char const*>(char const*): error: undefined reference to 'IProcessor<char const*>* CreateProcessor<char const*>()'
collect2: error: ld returned 1 exit status

我的想法是,当我们解决时CreateProcessorImplemented<char const*>,decltype(CreateProcessor<const char*>())不会失败,因为有一个模板原型,IProcessor<T> CreateProcessor()并且编译器认为decltype等于IProcessor<T>那是某种逻辑但不是我需要的.

Answer 1

使其工作的一种方法是使用包装器结构来CreateProcessor像这样工作:

#include <cstdio>
#include <type_traits>

// A workaround for void_t as described here: http://en.cppreference.com/w/cpp/types/void_t.
template<typename... Ts> struct make_void { typedef void type;};
template<typename... Ts> using void_t = typename make_void<Ts...>::type;

// An interface for a processor that receives a value of specific type.
template<class T>
class IProcessor {
public:
    virtual void process(T value) = 0;
};

// A processor for int.
class IntProcessor : public IProcessor<int> {
public:
    virtual void process(int value) override {
        printf("IntProcessor::process is called for value = %d\n", value);
    }
};

// Template prototype.
template<class T>
struct ProcessorCreator: std::false_type { 
   static IProcessor<T>* CreateProcessor();
};

// Template specialization for int.
template<>
struct ProcessorCreator<int>: std::true_type {
static IProcessor<int>* CreateProcessor() {
    return new IntProcessor();
}
};

// Detector of CreateProcessor.
template<class, class=void>
struct CreateProcessorImplemented : std::false_type { };

template<class T>
struct CreateProcessorImplemented<T, typename std::enable_if<ProcessorCreator<T>::value>::type > : std::true_type { };


// Specializations depending on existence of CreateProcessor.
template <typename T>
typename std::enable_if<CreateProcessorImplemented<T>::value, void>::type Process(T value) {
    IProcessor<T>* processor = ProcessorCreator<T>::CreateProcessor();
    processor->process(value);
}

template <typename T>
typename std::enable_if<!CreateProcessorImplemented<T>::value, void>::type Process(T value) {
    printf("Processor for requested typename is unavailable\n");
}


int main() {
    Process(42);
    Process("abc");

// static_assert(!CreateProcessorImplemented<char const*>::value, ":(");
/* This static_assert fails with an error:
 * code.cpp:56:5: error: static assertion failed: :(
 *      static_assert(!CreateProcessorImplemented<char const*>::value, ":(");
 */
}

或者,您可以删除模板声明并使用函数重载传递IProcessor模板参数类型 - 通过创建伪参数:

#include <cstdio>
#include <type_traits>

// A workaround for void_t as described here: http://en.cppreference.com/w/cpp/types/void_t.
template<typename... Ts> struct make_void { typedef void type;};
template<typename... Ts> using void_t = typename make_void<Ts...>::type;

// An interface for a processor that receives a value of specific type.
template<class T>
class IProcessor {
public:
    virtual void process(T value) = 0;
};

// A processor for int.
class IntProcessor : public IProcessor<int> {
public:
    virtual void process(int value) override {
        printf("IntProcessor::process is called for value = %d\n", value);
    }
};


IProcessor<int>* CreateProcessor(const int&) {
    return new IntProcessor();
}

// Detector of CreateProcessor.
template<class, class=void>
struct CreateProcessorImplemented : std::false_type { };

template<class T>
struct CreateProcessorImplemented<T, void_t<decltype(CreateProcessor(std::declval<T>()))>> : std::true_type { };


// Specializations depending on existence of CreateProcessor.
template <typename T>
typename std::enable_if<CreateProcessorImplemented<T>::value, void>::type Process(T value) {
    IProcessor<T>* processor = CreateProcessor(value);
    processor->process(value);
}

template <typename T>
typename std::enable_if<!CreateProcessorImplemented<T>::value, void>::type Process(T value) {
    printf("Processor for requested typename is unavailable\n");
}


int main() {
    Process(42);
    Process("abc");

// static_assert(!CreateProcessorImplemented<char const*>::value, ":(");
/* This static_assert fails with an error:
 * code.cpp:56:5: error: static assertion failed: :(
 *      static_assert(!CreateProcessorImplemented<char const*>::value, ":(");
 */
}