[英]Why can't I declare a type of friend function by template parameter but can with alias
[英]How can I make a typedef or alias for a template function that can also be used in friend function declarations?
如果我有這樣的事情:
#ifndef MYTEMPLATES_H
#define MYTEMPLATES_H
#include <iostream>
#include <memory>
#include <string>
class BaseClass;
// Alias (non-working) version:
//class DerivedClass1;
namespace MyTemplates
{
typedef std::unique_ptr<BaseClass> object_up;
template<typename SomeDerivedClass>
object_up createFromIntFloat(int intParam, float floatParam)
{
SomeDerivedClass *newObject_raw = new SomeDerivedClass(intParam, floatParam);
object_up uniqueObject {newObject_raw};
std::cout << "createFromIntFloat." << "\n";
return uniqueObject;
}
// Alias (non-working) version:
//using createFromIntFloatPtrType = object_up(*)(int intParam, float floatParam);
template<typename SomeDerivedClass>
object_up createFromString(std::string stringParam)
{
SomeDerivedClass *newObject_raw = new SomeDerivedClass(stringParam);
object_up uniqueObject {newObject_raw};
std::cout << "createFromString." << "\n";
return uniqueObject;
}
// Alias (non-working) version:
//using createFromStringPtrType = object_up(*)(std::string stringParam);
// Alias (non-working) version:
// createFromStringPtrType createDerivedClass1 = createFromString<DerivedClass1>;
}
#endif // MYTEMPLATES_H
#ifndef BASECLASS_H
#define BASECLASS_H
class BaseClass
{
protected:
BaseClass(){};
};
#endif // BASECLASS_H
#ifndef DERIVEDCLASS1_H
#define DERIVEDCLASS1_H
#include "baseclass.h"
#include "mytemplates.h"
class DerivedClass1 : public BaseClass
{
std::string stringParam;
protected:
DerivedClass1(std::string aStringParam) : BaseClass(), stringParam(aStringParam) {};
// Alias (non-working) version:
// friend MyTemplates::object_up MyTemplates::createDerivedClass1(std::string stringParam);
// Non-alias version:
friend MyTemplates::object_up MyTemplates::createFromString<DerivedClass1>(std::string stringParam);
};
#endif // DERIVEDCLASS1_H
#include "mytemplates.h"
#include "derivedclass1.h"
int main(int argc, char *argv[])
{
// Alias (non-working) version:
// MyTemplates::object_up uniqueClass1 = MyTemplates::createDerivedClass1("creation info for class 1");
// Non-alias version:
MyTemplates::object_up uniqueClass1 = MyTemplates::createFromString<DerivedClass1>("creation info for class 1");
}
我想在上面的代碼上創建某種別名,比如Alias(非工作)版本,這樣我就可以在main.cpp中的直接調用和在derivedclass1.h中的友元聲明中使用這個別名 - createDerivedClass1
如何將上面的代碼從非別名版本更改為別名版本並使其工作?
別名用於類型,因為類型不是 C++ 中的第一個 class 對象。 但是您不需要普通函數的別名,因為您可以簡單地使用 function 指針。
但是 function 指針只是一個指針,您必須聲明真正的 function 才能成為朋友。 這一切都歸結為:
...
class DerivedClass1 : public BaseClass
{
std::string stringParam;
protected:
DerivedClass1(std::string aStringParam) : BaseClass(), stringParam(aStringParam) {};
// Alias (non-working) version:
//friend MyTemplates::object_up MyTemplates::createDerivedClass1(std::string stringParam);
// Non-alias version:
friend MyTemplates::object_up MyTemplates::createFromString<DerivedClass1>(std::string stringParam);
};
namespace MyTemplates {
object_up(*createDerivedClass1)(std::string) = createFromString<DerivedClass1>;
}
int main(int argc, char* argv[])
{
// Alias (non-working) version:
MyTemplates::object_up uniqueClass1 = MyTemplates::createDerivedClass1("creation info for class 1");
// Non-alias version:
MyTemplates::object_up uniqueClass2 = MyTemplates::createFromString<DerivedClass1>("creation info for class 1");
}
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