字符常量与模板匹配问题

以下代码运行输出：

 

 A(const T&)

 

#include <iostream>#include <cstddef>template<class T>class A {public:A(const T& t) :val_(t){std::cout << "A(const T&)" << std::endl;}private:const T& val_;};template<>class A<const char *> {public:A(const char* t) :val_(t){std::cout << "A(const char*)" << std::endl;}private:const std::string val_;};template<class T>void foo(const T& t) {A<T> bar(t);}int main(){foo("Hello");system("pause");return 0;}

 

foo( "Hello" ); 并没有匹配偏特化的模板template<> class A<const char*>，这是因为"Hello"是一个字符常量，其类型是 const char[6]，所以不能与这个模板要求的模板参数匹配。要正确匹配则需要将const char[6]转换到const char*。

 

如下代码则可以正确解析：

 

#include <iostream>#include <cstddef>// compile-time type transformationtemplate<typename T>struct ptr_decay {typedef T type;};template<typename T>struct ptr_decay<T[]> {typedef T*type;};template<typename T, std::size_t N>struct ptr_decay<T[N]> {typedef T*type;};template<typename T>class A {public:A(){std::cout << "A<T>::A()/n";}};template<>class A<const char *> {public:A(){std::cout << "A<const char*>::A()/n";}};template<typename T>void foo(const T& t){// adjust type manually (array-to-pointer conversion) ...typedef typename ptr_decay<const T>::type decayed_t;A<decayed_t> a; // note: ^^^^^ (added const)}int main(){foo("Hello");system("pause");return 0;}