如何明确实例在C ++ MPL向量的所有成员的模板？

本文介绍了如何明确实例在C ++ MPL向量的所有成员的模板？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

考虑以下头文件：

// Foo.h class Foo { public: template <typename T> void read(T& value); };

我要明确地实例化在所有类型的源文件的Foo ::阅读成员函数模板包含在的boost :: MPL： ：矢量：

// Foo.cc #include <boost/mpl/vector.hpp> #include <boost/mpl/begin_end.hpp> #include "Foo.h" template <typename T> void Foo::read(T& value) { /* do something */ } typedef boost::mpl::vector<int, long, float> types; // template Foo::read<int >(int&); // template Foo::read<long >(long&); // template Foo::read<float>(float&); // instantiate automatically ???

这可能吗？在此先感谢，丹尼尔。

Is it possible? Thanks in advance, Daniel.

修改

我找到了一些解决方案 - 它似乎分配一个指针富::阅读＆LT; T＆GT; 在一个结构的构造，它的变量随后宣布，因实例：

I found some solution - it seems that assigning a pointer to Foo::read<T> in the constructor of a struct, of which variable is then declared, cause instantiation:

// intermezzo template <typename T> struct Bar { Bar<T>() { void (Foo::*funPtr)(T&) = &Foo::read<T>; } }; static Bar<int > bar1; static Bar<long > bar2; static Bar<float> bar3;

于是该过程可以automatized如下：

So then the process can be automatized as follows:

// Foo.cc continued template <typename B, typename E> struct my_for_each { my_for_each<B, E>() { typedef typename B::type T; // vector member typedef void (Foo::*FunPtr)(T&); // pointer to Foo member function FunPtr funPtr = &Foo::read<T>; // cause instantiation? } my_for_each<typename boost::mpl::next<B>::type, E> next; }; template<typename E> struct my_for_each<E, E> {}; static my_for_each< boost::mpl::begin<types>::type, boost::mpl::end<types>::type > first;

但我不知道，如果这个解决方案是便携式和符合标准的？ （与英特尔和GNU编译器。）

But I don't know if this solution is portable and standard-conformant? (Works with Intel and GNU compilers.)

推荐答案

如果你打算使用你的类只在单个模块（即你不会导出），可以使用升压/ MPL /的for_each。这种方式定义（使用MPL /的for_each）模板功能不会被导出（即使你申报类名或函数签名之前__declspec（出口））：

If you intend to use your class only in a single module (i.e. you won't export it) you can use boost/mpl/for_each. The template function defined this way (using mpl/for_each) won't be exported (even if you declare __declspec(export) before class name or function signature):

// Foo.cpp #include <boost/mpl/vector.hpp> #include <boost/mpl/for_each.hpp> template<class T> void read(T& value) { ... } using types = boost::mpl::vector<long, int>; //template instantiation struct call_read { template <class T> void operator()(T) { T t; //You should make sure that T can be created this way ((Foo*)nullptr)->read<T>(t); //this line tells to compiler with templates it should instantiate } }; void instantiate() { boost::mpl::for_each<types>(call_read()); }

如果你需要导出/导入您结构和模板方法存在使用升压/ preprocessor解决方案

If you need export/import you structure and template methods there is the solution using boost/preprocessor

// Foo.h #ifdef <preprocessor definition specific to DLL> # define API __declspec(dllexport) #else # define API __declspec(dllimport) #endif class API Foo { public: template<class T> void read(T& value); }; // Foo.cpp #include <boost/preprocessor/seq/for_each.hpp> #include <boost/preprocessor/seq/enum.hpp> #include <boost/mpl/vector.hpp> template<class T> void read(T& value) { ... } //using this macro you can define both boost::mpl structure AND instantiate explicitly your template function #define VARIANT_LIST (std::wstring)(long)(int) using types = boost::mpl::vector<BOOST_PP_SEQ_ENUM(VARIANT_LIST)>; //Here we should use our API macro #define EXPLICIT_INSTANTIATION(r, d, __type__) \ template API void Foo::read<__type__>(__type__&); BOOST_PP_SEQ_FOR_EACH(EXPLICIT_INSTANTIATION, _, VARIANT_LIST)

如果您不需要这个额外的功能的第一个解决方案是干净多了我觉得

If you don't need this extra functionality the first solution is much cleaner I guess