C++ polymorphism Virtual Function 多态 虚函数

Polymorphism in C++ https://www.tutorialspoint.com/cplusplus/cpp_polymorphism.htm

https://github.com/mongodb/mongo/blob/410656e971aff8f491a87337a17d04bd866389ba/src/mongo/base/initializer.cpp

/**
 *    Copyright (C) 2018-present MongoDB, Inc.
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the Server Side Public License, version 1,
 *    as published by MongoDB, Inc.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    Server Side Public License for more details.
 *
 *    You should have received a copy of the Server Side Public License
 *    along with this program. If not, see
 *    <http://www.mongodb.com/licensing/server-side-public-license>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the Server Side Public License in all respects for
 *    all of the code used other than as permitted herein. If you modify file(s)
 *    with this exception, you may extend this exception to your version of the
 *    file(s), but you are not obligated to do so. If you do not wish to do so,
 *    delete this exception statement from your version. If you delete this
 *    exception statement from all source files in the program, then also delete
 *    it in the license file.
 */

#include "mongo/platform/basic.h"

#include "mongo/base/initializer.h"

#include <iostream>

#include "mongo/base/deinitializer_context.h"
#include "mongo/base/global_initializer.h"
#include "mongo/base/initializer_context.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/quick_exit.h"

namespace mongo {

Initializer::Initializer() {}
Initializer::~Initializer() {}

Status Initializer::executeInitializers(const InitializerContext::ArgumentVector& args,
                                        const InitializerContext::EnvironmentMap& env) {
    std::vector<std::string> sortedNodes;
    Status status = _graph.topSort(&sortedNodes);
    if (Status::OK() != status)
        return status;

    InitializerContext context(args, env);

    for (size_t i = 0; i < sortedNodes.size(); ++i) {
        InitializerDependencyNode* node = _graph.getInitializerNode(sortedNodes[i]);

        // If already initialized then this node is a legacy initializer without re-initialization
        // support.
        if (node->isInitialized())
            continue;

        auto const& fn = node->getInitializerFunction();
        if (!fn) {
            return Status(ErrorCodes::InternalError,
                          "topSort returned a node that has no associated function: \"" +
                              sortedNodes[i] + '"');
        }
        try {
            status = fn(&context);
        } catch (const DBException& xcp) {
            return xcp.toStatus();
        }

        if (Status::OK() != status)
            return status;

        node->setInitialized(true);
    }
    return Status::OK();
}

Status Initializer::executeDeinitializers() {
    std::vector<std::string> sortedNodes;
    Status status = _graph.topSort(&sortedNodes);
    if (Status::OK() != status)
        return status;

    DeinitializerContext context{};

    // Execute deinitialization in reverse order from initialization.
    for (auto it = sortedNodes.rbegin(), end = sortedNodes.rend(); it != end; ++it) {
        InitializerDependencyNode* node = _graph.getInitializerNode(*it);
        auto const& fn = node->getDeinitializerFunction();
        if (fn) {
            try {
                status = fn(&context);
            } catch (const DBException& xcp) {
                return xcp.toStatus();
            }

            if (Status::OK() != status)
                return status;

            node->setInitialized(false);
        }
    }
    return Status::OK();
}

Status runGlobalInitializers(const InitializerContext::ArgumentVector& args,
                             const InitializerContext::EnvironmentMap& env) {
    return getGlobalInitializer().executeInitializers(args, env);
}

Status runGlobalInitializers(int argc, const char* const* argv, const char* const* envp) {
    InitializerContext::ArgumentVector args(argc);
    std::copy(argv, argv + argc, args.begin());

    InitializerContext::EnvironmentMap env;

    if (envp) {
        for (; *envp; ++envp) {
            const char* firstEqualSign = strchr(*envp, '=');
            if (!firstEqualSign) {
                return Status(ErrorCodes::BadValue, "malformed environment block");
            }
            env[std::string(*envp, firstEqualSign)] = std::string(firstEqualSign + 1);
        }
    }

    return runGlobalInitializers(args, env);
}

Status runGlobalDeinitializers() {
    return getGlobalInitializer().executeDeinitializers();
}

void runGlobalInitializersOrDie(int argc, const char* const* argv, const char* const* envp) {
    Status status = runGlobalInitializers(argc, argv, envp);
    if (!status.isOK()) {
        std::cerr << "Failed global initialization: " << status << std::endl;
        quickExit(1);
    }
}

}  // namespace mongo

　　

#include <iostream>
using namespace std;

class Shape {
   protected:
      int width, height;

   public:
      Shape( int a = 0, int b = 0){
         width = a;
         height = b;
      }
      int area() {
         cout << "Parent class area :" <<endl;
         return 0;
      }
};
class Rectangle: public Shape {
   public:
      Rectangle( int a = 0, int b = 0):Shape(a, b) { }

      int area () {
         cout << "Rectangle class area :" <<endl;
         return (width * height);
      }
};

class Triangle: public Shape {
   public:
      Triangle( int a = 0, int b = 0):Shape(a, b) { }

      int area () {
         cout << "Triangle class area :" <<endl;
         return (width * height / 2);
      }
};

// Main function for the program
int main() {
   Shape *shape;
   Rectangle rec(10,7);
   Triangle  tri(10,5);

   // store the address of Rectangle
   shape = &rec;

   // call rectangle area.
   shape->area();

   // store the address of Triangle
   shape = &tri;

   // call triangle area.
   shape->area();

   return 0;
}

　　Polymorphism in C++ https://www.tutorialspoint.com/cplusplus/cpp_polymorphism.htm

C++ 多态 | 菜鸟教程 http://www.runoob.com/cplusplus/cpp-polymorphism.html