指针的C ++模板专业化？ [英] C++ template specialization for pointer?
问题描述
我读了书< C ++模板-完整指南>和所学指针的模板专门知识。 （也许我误会了本书的这一部分）
I read book < C++ Templates - the Complete Guide > and learned template specialization for pointer. (maybe I misunderstand this part of the book)
（1）这是我的简单模板：
(1) Here's my simple template:
#include <iostream>
template<typename T>
void Function(const T& a)
{
std::cout << "Function<T>: " << a << std::endl;
}
template<typename T>
void Function<T*>(const T* a)
{
std::cout << "Function<T*>: " << a << std::endl;
}
int main(void)
{
Function(1);
Function(1.2);
Function("hello");
Function((void*)0x25);
return 0;
}
我使用ubuntu16.04 x64，g ++ 5.3，编译器报告：
I use ubuntu16.04 x64, g++ 5.3, the compiler report:
$ g++ main.cpp -o main.exe
main.cpp:10:29: error: non-type partial specialization ‘Function<T*>’ is not allowed
void Function<T*>(const T* a)
（2），但此代码正确：
(2) but this code is correct:
#include <iostream>
template<typename T>
void Function(const T& a)
{
std::cout << "Function<T>: " << a << std::endl;
}
int main(void)
{
Function(1);
Function(1.2);
Function("hello");
Function((void*)0x25);
return 0;
}
结果显示：
$ g++ main.cpp -o main.exe
$ ./main.exe
Function<T>: 1
Function<T>: 1.2
Function<T>: hello
Function<T>: 0x25
我的问题是：关于指针专门化的书是错误的吗？还是我不明白书中这部分的含义？还是其他？
My question is: Is the book about pointer specialization is wrong ? Or I mis understand the meaning of this part in the book ? Or something else ?
有关类中指针专门化的更新。
Update about pointer specialization in class.
（3）具有指针专门化的模板类：
(3) template class with pointer specialization:
#include <iostream>
template<typename T>
struct Base {
T member;
Base(const T& a)
: member(a)
{
}
void hello()
{
std::cout << member << std::endl;
}
};
template<typename T>
struct Base<T*> {
T* member;
Base(T* a)
: member(a)
{
}
void hello()
{
std::cout << member << std::endl;
}
};
int main(void)
{
Base<int> b1(12);
Base<double> b2(2.4);
Base<char*> b3("hello");
Base<void*> b4((void*)0x25);
b1.hello();
b2.hello();
b3.hello();
b4.hello();
return 0;
}
此代码正确，但有一个警告：
this code is correct with one warning:
$ g++ main.cpp -o main.exe
main.cpp: In function ‘int main()’:
main.cpp:37:27: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
Base<char*> b3("hello");
^
$ ./main.exe
12
2.4
hello
0x25
（4）没有指针专门化的模板类：
(4) template class without pointer specialization:
#include <iostream>
template<typename T>
struct Base {
T member;
Base(const T& a)
: member(a)
{
}
void hello()
{
std::cout << member << std::endl;
}
};
int main(void)
{
Base<int> b1(12);
Base<double> b2(2.4);
Base<char*> b3("hello");
Base<void*> b4((void*)0x25);
b1.hello();
b2.hello();
b3.hello();
b4.hello();
return 0;
}
结果是相同的：
$ g++ main.cpp -o main.exe
main.cpp: In function ‘int main()’:
main.cpp:39:27: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
Base<char*> b3("hello");
^
$ ./main.exe
12
2.4
hello
0x25
这是否意味着不需要指针专门化？
或此功能在不同的编译器上的行为可能有所不同吗？
Does this means pointer specialization is needless ? Or maybe this feature behave differently on different compiler ?
推荐答案
错误消息告诉您出了什么问题：
The error message told you what is wrong:
non-type partial specialization ‘Function<T*>’ is not allowed
您只能部分专门化类型（类）。您尝试了部分专门化功能。函数不是类型；您只能完全专门化它们。
You can only partially specialize types (classes). You've tried to partially specialize a function. Functions are not types; you can only fully specialize them.
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