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我们都知道C有静态多态和动态多态,动态多态通过虚函数表实现,他的缺点就是对效率产生一点点影响
可以用CRTP解决这个问题
我们先举一个动态多态的例子:
#include iostream
using namespace std;class …Curiously Recurring Template Pattern 奇怪的模板递归 — C20
我们都知道C有静态多态和动态多态,动态多态通过虚函数表实现,他的缺点就是对效率产生一点点影响
可以用CRTP解决这个问题
我们先举一个动态多态的例子:
#include iostream
using namespace std;class Base
{
public:virtual void invoking(){cout Base endl;}
};class Derived1 final : public Base
{
public:virtual void invoking() override final{cout Derived1 endl;}
};class Derived2 final : public Base
{
public:virtual void invoking() override final{cout Derived2 endl;}
};int main()
{Derived1* derived1 new Derived1;Derived2* derived2 new Derived2;derived1-invoking();derived2-invoking();}会在运行时查找虚函数表,影响运行时效率
使用CRTP编译器多态
特点:子类继承基类,且基类有一个模板参数,并以子类类型为参数
#include iostream
using namespace std;template typename Derived
class Base
{
public:void interface(){static_castDerived*(this)-implementation();}void implementation(){cout Implementation Base endl;}
};class Derived1 :public BaseDerived1
{
public:void implementation(){cout Implementation Derived1 endl;}
};class Derived2 : public BaseDerived2
{
public:void implementation(){cout Implementation Derived2 endl;}
};class Derived3 : public BaseDerived3
{
public:
};template typename T
void execute(T base)
{base.interface();
}int main(int argc, char* argv[])
{Derived1 d1;execute(d1);Derived2 d2;execute(d2);Derived3 d3;execute(d3);
}
每个基类都调用 base.interface(); 然后调用static_castDerived*(this)-implementation();
这样就在编译器实现了多态,避免了动态多态
1class Baseclass Derived1 size(1):
1 ---
1 ---
1class Baseclass Derived2 size(1):
1 ---
1 ---
1class Baseclass Derived3 size(1):
1 ---
1 ---#include chrono
#include iostreamauto start std::chrono::steady_clock::now();void writeElapsedTime() {auto now std::chrono::steady_clock::now();std::chrono::durationdouble diff now - start;std::cerr diff.count() sec. elapsed: ;
}template typename ConcreteMessage // (1)
struct MessageSeverity {void writeMessage() { // (2)static_castConcreteMessage*(this)-writeMessageImplementation();}void writeMessageImplementation() const {std::cerr unexpected std::endl;}
};struct MessageInformation : MessageSeverityMessageInformation {void writeMessageImplementation() const { // (3)std::cerr information std::endl;}
};struct MessageWarning : MessageSeverityMessageWarning {void writeMessageImplementation() const { // (4)std::cerr warning std::endl;}
};struct MessageFatal : MessageSeverityMessageFatal {}; // (5)template typename T
void writeMessage(T messServer) {writeElapsedTime();messServer.writeMessage(); // (6)}int main() {std::cout std::endl;MessageInformation messInfo;writeMessage(messInfo);MessageWarning messWarn;writeMessage(messWarn);MessageFatal messFatal;writeMessage(messFatal);std::cout std::endl;}Info; writeMessage(messInfo);
MessageWarning messWarn;
writeMessage(messWarn);MessageFatal messFatal;
writeMessage(messFatal);std::cout std::endl;} 
