【c++11】cpp实现模板函数的声明与定义分离

一、分离模板的声明与定义方法一

在模板定义的cpp文件中声明其特化版本

other.h

#pragma once

#include 
#include 

using namespace std;

template<typename T>
void show(T t);

oher.cpp

#include "other.h"
template<typename T>
void show(T t)
{
	cout<<"t"<<'\n';
}


template void show<int>(int);
template void show<double>(double);
template void show<string>(string);

二、分离模板的声明与定义方法二

使用variant(C++17语法)代替上述的方式

other.h

#pragma once
#include 
#include 
#include 

using namespace std;

void show(variant<int , double, string> t);

other.cpp

#include "other.h"

void show(variant<int , double, string> t)
{
	cout << t.index() <<endl;
/*
visit内部的实现方式可能就是通过index()选择的；
auto visitor = [](auto&& arg){cout<(t));
}
else if (t.index() == 1)
{
	visitor(get(t));
}
else if(t.index() == 2)
{
	visitor(get(t));
}

or
if (holds_alternative(t))
{
	visitor(get(t));
}
else if (holds_alternative(t))
{
	visitor(get(t));
}
else if(holds_alternative(t))
{
	visitor(get(t));
}
*/


	visit([](auto&& arg){cout<<args<<endl;},t);
}

解决了分离模板的声明与实现后，如果想要把每个variant对象保存至vector，出现的问题是：每个variant=32+8，string bytes+index bytes。每次占用内存过大，需要优化。所以使用data-oriented design进行改造

other.h

#pragma once

#include 
#include 

using namespace std;

using Object = variant<int, double, string>;

void add_object(Object o);
void print_objects();

other.cpp

#include "other.h"
#Include <vector>

static vector<variant<int.double.string>> objects;
static vector<int> int_objects;
static vector<double> double_objects;
static vector<string> string_objects;

static constexpr int v = sizeof(variant<int.double.string>);

void add_object(variant<int.double.string> o)
{
	objects.push_back(o);objects
}


void print_objects()
{
	for(auto&& o : objects)
	{
		visit([](auto&& arg){std::cout<<arg<<std::endl;},o);
	}
}

other.cpp

#include "other.h"
#Include <vector>

static vector<int> int_objects;
static vector<double> double_objects;
static vector<string> string_objects;


void add_object(variant<int,double,string> o)
{
	if(holds_alternatibe<int>(o))
	{
		int_objects.push_back(get<int>(o));
	}
	else if(holds_alternatibe<double>(o))
	{
		double_objects.push_back(get<double>(o));
	}
	else if(holds_alternatibe<string>(o))
	{
		string_objects.push_back(get<string>(o));
	}
}

void print_objects()
{
	for(auto&& o: int_objects)
	{
		cout<<o<<endl;
	}
	for(auto&& o: double_objects)
	{
		cout<<o<<endl;
	}
	for(auto&& o: string_objects)
	{
		cout<<o<<endl;
	}
}

使用编译器for循环以及tuple简化代码

static tuple<vector<int>. vector<double>, vector<string>> objects;

void add_object(variant<int, double, string> o)
{
	if (o.index() == 0)
	{
		std::get<0>(objects).push_back(std::get<0>(o));
	}

	else if (o.index() == 1)
	{
		std::get<1>(objects).push_back(std::get<1>(o));
	}

	else 	if (o.index() == 2)
	{
		std::get<2>(objects).push_back(std::get<2>(o));
	}
}


void print_objects()
{
	for(auto&& o: std::get<0>(objects))
	{
		cout<<o<<endl;
	}
	for(auto&& o: std::get<1>(objects))
	{
		cout<<o<<endl;
	}
	for(auto&& o: std::get<2>(objects))
	{
		cout<<o<<endl;
	}
}

other.cpp

static tuple<vector<int>. vector<double>, vector<string>> objects;

template<int N>
struct int_constant{
	static constexpr int value = N;
};

template <int N, typename Lambda>
void static_for(Lambda&& lambda)
{
	if constexpr (N > 0)
	{
		static_for<N-1>(lambda);
		int_constant<N-1> ic;
		lambda(ic);
	}
}

void add_objects(variant<int, double, string> o)
{
	static_for<3>([&](auto ic){
		std::get<ic.value>(objects).push_vack(std::get<ic.value>(o));
	});
}

void print_objects()
{
	static_for<3>([&](auto ic){
		for(auto&& o: std::get<ic.value>(objects))
		{
			cout<<o<<endl;
		}
	});
}

取出variant中的所有类型，以vector,vector的形式放在tuple中的类型中。即Object类型->Objects类型

other.cpp

template <class V>
struct variant_to_tuple_of_vector
{
}

template <class ...Ts>
struct variant_to_tuple_of_vector<variant<Ts...>> {
    using type = tuple<vector<Ts>...>;
};

static variant_to_tuple_of_vector<Object>::type objects;

自动提取variant中所有参数的个数，优化lambda只在输入的variant变量中插入

other.cpp

void add_objects(variant<int, double, string> o)
{
	static_for<std::variant_size_v<decltype(o)>>([&](auto ic){
		if (o.index() == ic.value)
		{
			std::get<ic.value>(objects).push_vack(std::get<ic.value>(o));
		}
		
	});
}


void print_objects()
{
	static_for<std::variant_size_v<decltype(o)>([&](auto ic){
		for(auto&& o: std::get<ic.value>(objects))
		{
			cout<<o<<endl;
		}
	});
}

参考

• 【C++模板课】闭源软件能否元编程？
• code