解密和升压：：变量-检索当前值

Question

我有以下代码：

#include <boost/variant.hpp>

#include <iostream>
#include <string>

boost::variant<int, double, std::string> variant;

template <typename FirstArg, typename... OtherArgs>
auto bar(const std::type_info& variant_type_info) -> decltype(typeid(FirstArg) == variant_type_info ? boost::get<FirstArg>(variant) : bar<OtherArgs...>(variant_type_info))
{
    if (typeid(FirstArg) == variant_type_info)
    {
        return boost::get<FirstArg>(variant);
    }

    return bar<OtherArgs...>(variant_type_info);
}

template <typename... VariantArgs>
auto foo(const boost::variant<VariantArgs...>& variant) -> decltype(bar<VariantArgs...>(variant.type()))
{
    return bar<VariantArgs...>(variant.type());
}

int main()
{
    variant = 0.5;
    const auto& baz = foo(variant);
    std::cout << baz << '\n';
}

它给我带来了以下错误：

main.cpp:9:135: error: 'bar' was not declared in this scope

 auto bar(const std::type_info& variant_type_info) -> decltype(typeid(FirstArg) == variant_type_info ? boost::get<FirstArg>(variant) : bar<OtherArgs...>(variant_type_info))

                                                                                                                                       ^

main.cpp:9:148: error: expected primary-expression before '...' token

 auto bar(const std::type_info& variant_type_info) -> decltype(typeid(FirstArg) == variant_type_info ? boost::get<FirstArg>(variant) : bar<OtherArgs...>(variant_type_info))

                                                                                                                                                    ^

main.cpp:9:148: error: expected ')' before '...' token

main.cpp:9:135: error: 'bar' was not declared in this scope

 auto bar(const std::type_info& variant_type_info) -> decltype(typeid(FirstArg) == variant_type_info ? boost::get<FirstArg>(variant) : bar<OtherArgs...>(variant_type_info))

                                                                                                                                       ^

main.cpp:9:54: error: expected type-specifier before 'decltype'

 auto bar(const std::type_info& variant_type_info) -> decltype(typeid(FirstArg) == variant_type_info ? boost::get<FirstArg>(variant) : bar<OtherArgs...>(variant_type_info))

                                                      ^

main.cpp:9:54: error: expected initializer before 'decltype'

main.cpp:20:69: error: 'bar' was not declared in this scope

 auto foo(const boost::variant<VariantArgs...>& variant) -> decltype(bar<VariantArgs...>(variant.type()))

                                                                     ^

main.cpp:20:69: error: 'bar' was not declared in this scope

main.cpp:20:84: error: expected primary-expression before '...' token

 auto foo(const boost::variant<VariantArgs...>& variant) -> decltype(bar<VariantArgs...>(variant.type()))

                                                                                    ^

main.cpp:20:84: error: expected ')' before '...' token

main.cpp:20:69: error: 'bar' was not declared in this scope

 auto foo(const boost::variant<VariantArgs...>& variant) -> decltype(bar<VariantArgs...>(variant.type()))

                                                                     ^

main.cpp:20:69: error: 'bar' was not declared in this scope

main.cpp:20:60: error: expected type-specifier before 'decltype'

 auto foo(const boost::variant<VariantArgs...>& variant) -> decltype(bar<VariantArgs...>(variant.type()))

                                                            ^

main.cpp:20:60: error: expected initializer before 'decltype'

main.cpp: In function 'int main()':

main.cpp:28:34: error: 'foo' was not declared in this scope

     const auto& baz = foo(variant);

Coliru示例- http://coliru.stacked-crooked.com/a/4467c33489b08359

我发现我不能在解密中引用相同的函数。这种情况有什么解决办法吗？我试图编写函数从boost::variant对象中检索当前值。

Answer 1

是的，在一般情况下有一个解决办法。它叫：

偏序要获得前向引用，请将方法移到类命名空间中。

然而，

这里什么也救不了你。除非您在编译时已经知道了参数的静态类型，否则不可能希望编译器恢复参数的静态类型。这应该是显而易见的。

我的意思是，foo()的返回类型应该是什么？它怎么可能包含所有可能的元素类型？正确的。这就是为什么你有变体的原因。

前进的三条道路：

1. 静态地了解类型： const & baz =boost：：get(变体)；
2. 返回类型为： std::type_info const& typeinfo_value = variant.type()；
3. 分支并创建单独的路径来处理所有情况。Boost为本例提供了static_visitor： 结构访问者: boost::static_visitor { std::string运算符()( std::string const&) const {返回" std::string "；}std：：string运算符(Int)(Int) const {int "int"；}std：：string运算符()()(Double) const {返回"double"；} template std::string运算符()(boost：：variant const& v) const {返回boost::apply_visitor(*this，v)；}；int main() {静态const访问者_visitor；变量= 0.5f；std::cout <<“实际类型为”<< _visitor(变体) << "\n"；}