将std :: map用作模板模板参数时出错 [英] Error when pass std::map as template template argument
问题描述
我定义了这样的函数,其中有一个模板模板类
I defined a function like this, in which there is a template template class
template<typename Key, typename Value, template <typename, typename> class Map>
struct ForEachOf {
void operator()(const Map<Key, Value>& map, std::function<void (Key, Value)> func) {
for(const auto& pair : map) {
func(pair.first, pair.second);
}
}
};
std::map<int, string> m { {1, "foo"}, {3, "bar"}};
ForEachOf<int, string, std::map> forEachOf;
forEachOf(m, [](int key, string value) {
cout << key << value;
});
但是,以上代码无法编译.错误是这样的:
However, above code is not able to compile. the error is like:
error: template template argument has different template parameters
than its corresponding template template parameter
ForEachOf<int, string, std::map> forEachOf;
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__tree:1119:5: note: too many template
parameters in template template argument
template <class, class, class, class> friend class _LIBCPP_TYPE_VIS_ONLY map;
note: previous template template parameter is here
template<typename Key, typename Value, template <typename, typename> class Map>
然后如何在此处传递std::map
作为模板template参数?
Then How to pass std::map
as the template template parameter here?
推荐答案
您的问题是 ::std::map
不只接受两个模板参数.
Your problem is that ::std::map
does not take only two template parameters.
解决方案只是添加一个仅包含两个参数而不是四个参数的模板:
The solution is simply to add a template that takes only two parameters instead of four:
template<typename key, typename value>
using mymap = std::map<key, value>;
(看到它)
或者,也可以添加缺少的参数及其默认值:
Or, alternatively add the missing arguments with their defaults:
template<typename Key, typename Value, template <typename, typename, typename, typename> class Map>
struct ForEachOf {
void operator()(const Map<Key, Value, ::std::less<Key>, ::std::allocator<std::pair<const Key, T> >>& map, std::function<void (Key, Value)> func) {
for(const auto& pair : map) {
func(pair.first, pair.second);
}
}
};
(看到它)
可以类似地通过使用可变参数类型模板来编写:
Which can similarly be written by using a variadic type template:
template<typename Key, typename Value, template <typename...> class Map>
struct ForEachOf {
void operator()(const Map<Key, Value>& map, std::function<void (Key, Value)> func) {
for(const auto& pair : map) {
func(pair.first, pair.second);
}
}
};
(看到它)
当然,您也可以只创建一个模板函数来获取地图并推导所有内容:
Of course, you could also just create a template function that takes the map and deduces everything:
#include <map>
#include <string>
#include <iostream>
template<typename T, typename F>
void ForEachOf(T&& map, F&& func) {
for(auto& pair : map) {
func(pair.first, pair.second);
}
}
int main(void) {
std::map<int, std::string> m { {1, "foo"}, {3, "bar"}};
ForEachOf(m, [](auto key, auto value) {
::std::cout << key << value;
});
}
(看到它)
这篇关于将std :: map用作模板模板参数时出错的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!