提升：爲什麼apply_visitor的不在此代碼

Question

我得到以下編譯器錯誤工作：

/usr/include/boost/variant/variant.hpp:832:32: error: no match for call to ‘(const StartsWith) (bool&)’

以下代碼。有人知道爲什麼嗎？

#include "boost/variant/variant.hpp" 
#include "boost/variant/apply_visitor.hpp" 

using namespace std; 
using namespace boost; 

typedef variant<bool, int, string, const char*> MyVariant; 

class StartsWith 
    : public boost::static_visitor<bool> 
{ 
public: 
    string mPrefix; 
    bool operator()(string &other) const 
    { 
     return other.compare(0, mPrefix.length(), mPrefix); 
    } 
    StartsWith(string const& prefix):mPrefix(prefix){} 
}; 

int main(int argc, char **argv) 
{ 
    MyVariant s1 = "hello world!"; 
    apply_visitor(StartsWith("hel"), s1); // << compiler error 
    return 0; 
} 

Answer 1

您必須爲在MyVariant中聲明的每種類型提供操作符。