boost::variant的诡异现象引发的思考

桃扇骨 2022-06-14 10:39 145阅读 0赞

#include <iostream>
    #include <string>
    #include <boost/optional.hpp>
    #include <boost/variant.hpp>
    #include <boost/any.hpp>
    #include <list>
    using namespace std;
    using namespace boost;
    struct MyPair
    {
    	template<typename T1,typename T2>
    	MyPair(const T1&& k, const T2&& v):key(k), value(v){}
    	template<typename T1, typename T2>
    	MyPair(const T1& k, const T2& v) : key(k), value(v) {}
    #if 1 //bug fix
    	template<typename T1> MyPair(const T1 &k, const char* v) : key(k), value(string(v)) {}
    	template<typename T2> MyPair(const char* k, const T2& v) : key(string(k)), value(v) {}
    	MyPair(const char* k, const char* v) : key(string(k)), value(string(v)) {}
    #endif
    	boost::variant<int, long, double, bool, string> key;
    	boost::variant<int, long, double, bool, string> value;
    };
    class test
    {
    	class my_visitor : public boost::static_visitor<>
    	{
    	public:
    		template<typename T1, typename T2>
    		void operator()(const T1 &key, const T2 &value) const
    		{
    			//cout << typeid(T1).name()<<":"<< typeid(T2).name() << endl;
    			cout << boost::get<T1>(key) << ":" << boost::get<T2>(value) << endl;
    		}	
    	};
    public:
    	test(const std::initializer_list<MyPair >& pairs)
    		:test(begin(pairs),end(pairs))
    	{
    	}
    	template <typename T>
    	test(const T& b,const T &d)
    	{
    		for_each(b, d, [](auto &t) {boost::apply_visitor(my_visitor(), t.key, t.value); });
    	}
    };
    int main()
    {
    	test t({ {"kl", "111"},{"k2", 123},{"k3",456} });
    	return 0;
    }

上代码，输出

kl:111
    k2:123
    k3:456

把fix出开关关闭 输出

1:1
    1:123
    1:456

猜猜为什么呢？

原来编译器优先把const char\*转换为bool，而不是string，参考链接

[https://stackoverflow.com/questions/13268608/boostvariant-why-is-const-char-converted-to-bool][https_stackoverflow.com_questions_13268608_boostvariant-why-is-const-char-converted-to-bool]

[https_stackoverflow.com_questions_13268608_boostvariant-why-is-const-char-converted-to-bool]: https://stackoverflow.com/questions/13268608/boostvariant-why-is-const-char-converted-to-bool