C++ 从 protected 抽象/派生类创建变量

Question

我想弄清楚从派生类创建变量时哪里出错了。我有抽象类、派生类，并试图在主测试程序中将派生类创建为变量。但是我得到错误:没有匹配函数调用 DerivedPlayer::DerivedPlayer()’。我一直无法找到创建和初始化派生类变量的正确语法。另请注意，抽象类的构造函数是 protected 。

抽象 header (Base.h)

    #ifndef BASE_H_
    #define BASE_H_

    #include <iostream>
    #include <vector>

    class Base {
      public:
        virtual ~Base() {}

      protected:
        Base(std::string s) : x(0), s(s), v(0) {}

        int x;
        std::string s;
        std::vector<int> v;
    };
    #endif

派生头文件(Derived.h)

    #ifndef DERIVED_H_
    #define DERIVED_H_

    #include "Base.h"

    class Derived : public Base {
    public:
        Derived(std::string name){ s = name; }
        virtual ~Derived();
    };

    #endif

测试代码(InTest.cpp)

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

    int main() {

        Derived a = Derived("R2-D2");
        Derived b = Derived("C-3PO");

        return 0;
    }

构建日志

    03:23:52 **** Incremental Build of configuration Debug for project InTest ****
    make all 
    Building file: ../src/InTest.cpp
    Invoking: GCC C++ Compiler
    g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/InTest.d" -MT"src/InTest.o" -o "src/InTest.o" "../src/InTest.cpp"
    In file included from ../src/InTest.cpp:2:0:
    ../src/Derived.h: In constructor ‘Derived::Derived(std::string)’:
    ../src/Derived.h:8:27: error: no matching function for call to ‘Base::Base()’
      Derived(std::string name){ s = name; }
                               ^
    ../src/Derived.h:8:27: note: candidates are:
    In file included from ../src/Derived.h:4:0,
                     from ../src/InTest.cpp:2:
    ../src/Base.h:12:2: note: Base::Base(std::string)
      Base(std::string s) : x(0), s(s), v(0) {}
      ^
    ../src/Base.h:12:2: note:   candidate expects 1 argument, 0 provided
    ../src/Base.h:7:7: note: Base::Base(const Base&)
     class Base {
           ^
    ../src/Base.h:7:7: note:   candidate expects 1 argument, 0 provided
    make: *** [src/InTest.o] Error 1

    03:23:52 Build Finished (took 214ms)

Answer 1

错误信息的主要部分如下:

../src/Derived.h: In constructor ‘Derived::Derived(std::string)’:
../src/Derived.h:8:27: error: no matching function for call to ‘Base::Base()’
  Derived(std::string name){ s = name; }

因为Derived继承自Base，每次构造Derived对象时，Base类的构造函数也有运行。您当前代码的问题是您让默认的 Base 构造函数被调用，但实际上没有。

您通过“调用”Derived 构造函数初始化列表中正确的Base 构造函数来解决它:

Derived::Derived(std::string name)
    : Base(name)
{}