第三方库隐藏实现

场景

当我们作为第三方开发者，需要提供库给客户使用，同时，我们又不想让用户看到我们内部的实现方法

UML

比如，我们提供给用户一个打印机类，用来实现打印服务

实现

printer.h

#ifndef _PRINT_H_
#define _PRINT_H_

#include <string>

class CPrinter
{
public:
    CPrinter();
    virtual ~CPrinter();
    
    static CPrinter* getInstance(const std::string & strFile, int iPageNum);
    virtual void Print() = 0;

private:
    static CPrinter * m_sInstance;
};


#endif

printer.cpp

#include "printer.h"
#include "printerimpl.h"

#include <iostream>

CPrinter * CPrinter::m_sInstance = nullptr;

CPrinter::CPrinter()
{
    std::cout << "CPrinter constructor" << '\n';
}

CPrinter::~CPrinter()
{
    std::cout << "CPrinter deconstructor" << '\n';
}

CPrinter* CPrinter::getInstance(const std::string & strFile, int iPageNum)
{
    if (m_sInstance == nullptr)
    {
        // 这里使用双检锁
        // lock
        m_sInstance = new CPrinterImpl(strFile, iPageNum);
        //unlock
    }

    return m_sInstance;
}

printerimpl.h

#ifndef _PRINTIMPL_H_
#define _PRINTIMPL_H_

#include "printer.h"
#include <string>

class CPrinterImpl : public CPrinter
{
public:
    CPrinterImpl(const std::string & strFile, int iPageNum);
    ~CPrinterImpl();
    void Print() override;
    
private:
    std::string m_strFile;
    int m_iPageNum;
};

#endif

printimpl.cpp

#include "printerimpl.h"
#include <iostream>

CPrinterImpl::CPrinterImpl(const std::string & strFile, int iPageNum) : m_strFile(strFile), m_iPageNum(iPageNum)
{
    std::cout << "CPrinterImpl constructor" << '\n';
}

CPrinterImpl::~CPrinterImpl()
{
    std::cout << "CPrinterImpl deconstructor" << '\n';
}

void CPrinterImpl::Print()
{
    std::cout << "print server in used" << '\n';
}

main.cpp

#include "printer.h"


int main()
{
    CPrinter* p = CPrinter::getInstance("programming", 98);

    p->Print();

    delete p;

    return 0;
}

Makefile

cc         := g++
solib     := libprinter.so
libname    := -lprinter
src       := printer.cpp printerimpl.cpp
obj        := $(src:.cpp=.o)
target    := a.out

.PHONY: all

all: build link

build: $(solib)
$(solib): $(obj)
    $(cc) -shared -o $@ $^
$(obj):%.o:%.cpp
    $(cc) -fPIC -c $< -o $@

link: $(target)
$(target): main.cpp $(solib)
    $(cc) main.cpp -o $@ -L. $(libname)

我们可以只提供给用户一个 printer.h 头文件和 libprinter.so 库文件，不会暴露更多的细节给用户