#include <string>
template
<
typename CharType,
template<class, class, class> class StringType = std::basic_string
<CharType, std::char_traits<CharType>, std::allocator<CharType>>
>
void f(CharType, StringType)
{}
int main()
{
char c;
std::string str;
f(c, str);
//
// error : default template argument for
// a template template parameter must be a class template
//
}
为什么不能默认template template parameter
?
尝试
template
<
typename CharType,
template<class, class, class> class StringType = std::basic_string
>
void f(CharType, StringType<CharType, std::char_traits<CharType>,
std::allocator<CharType>>)
{}
std::basic_string<CharType, std::char_traits<CharType>, std::allocator<CharType>>
是一个简单的打字名称;如果您想要template<typename, typename, typename> class
,则必须丢弃模板参数并使用骨架:std::basic_string
另外,这对StringType实现不太限制:
#include <string>
template
<
typename CharType,
typename StringType = std::basic_string<CharType,std::char_traits<CharType>,std::allocator<CharType>>
>
void f(CharType, StringType)
{
}
int main()
{
char c;
std::string str;
f(c, str);
const char* cstring;
f(c, cstring); // also works
}