我正在经历boost::variant，想知道如何进行跟踪工作？
typedef boost::variant<int,std::string> myval;
int main()
{
std::vector<myval> vec;
std::ifstream fin("temp.txt");
//How following can be achieved ?
std::copy(std::istream_iterator<myval>(fin), //Can this be from std::cin too ?
std::istream_iterator<myval>(),
std::back_inserter(vec));
}
对于类数据成员，我们可以选择重载>>运算符，但是该怎么做myval呢？
可以重载operator>>的variant，就像任何其他类型。但是，由您决定实现逻辑来决定从流中读取哪种类型并将其存储在中variant。这是您可能如何做的完整示例：
#include "boost/variant.hpp"
#include <iostream>
#include <cctype>
#include <vector>
#include <string>
typedef boost::variant<int, std::string> myval;
namespace boost { // must be in boost namespace to be found by ADL
std::istream& operator>>(std::istream& in, myval& v)
{
in >> std::ws; // throw away leading whitespace
int c = in.peek();
if (c == EOF) return in; // nothing to read, done
// read int if there's a minus or a digit
// TODO: handle the case where minus is not followed by a digit
// because that's supposed to be an error or read as a string
if (std::isdigit(static_cast<unsigned char>(c)) || c == '-') {
int i;
in >> i;
v = i;
} else {
std::string s;
in >> s;
v = s;
}
return in;
}
} // namespace boost
// visitor to query the type of value
struct visitor : boost::static_visitor<std::string> {
std::string operator()(const std::string&) const
{
return "string";
}
std::string operator()(int) const
{
return "int";
}
};
int main()
{
std::vector<myval> vec;
std::copy(
std::istream_iterator<myval>(std::cin),
std::istream_iterator<myval>(),
std::back_inserter(vec));
std::cout << "Types read:\n";
for (const auto& v : vec) {
std::string s = boost::apply_visitor(visitor(), v);
std::cout << s << '\n';
}
}
输入示例： 1 2 3 hello 4 world
输出：
Types read:
int
int
int
string
int
string
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