How do I call derived functions from base variables in C++?

I am currently working with a a class, and three other classes that derive from that base class. The base class is supposed to be essentially abstract, so that I can call the virtual function of a base class, and have it do the expected function of the derived class. My end goal is to do something like this:

class Car {  public:      virtual void drive() = 0;  // Pure-virtual function  };    class Ford : public Car {  public:      virtual void drive() { std::cout << "driving Ford\n"; }  };    

This should work. In an empty file, this setup does work. My implementation must be wrong, but the c++ compiler doesn't give me any good hints on what the error is.

Currently, I am using a function to create the derived class, returning the base class to be used in main. I've tried all I could think of:

• Passing in a pointer that won't be destroyed after you exit
• Returning the derived class and storing into the base class
• Casting a base class/pointer into the derived class
• Setting a dereferenced base class pointer to a derived class

I just could not get it to work for the life of me. The virtual function just causes a seg fault whenever run from a base class variable. Here is all of the code that I believe is relevant, in its current state, as I am testing.

Instruction.h:

class Instruction {      public:          virtual void print(){};          bool operator==(const int& rhs) const;          void writeBack();          void memory();          virtual void execute(long registers[32], long dmem[32]){};          unsigned int str2int(const char *str, int h);          virtual ~Instruction(){};  };  

ITypeInstruction.h:

  class IType: public Instruction {      private:          int op;          string label;          string rs;          string rt;          string immediate;          map<string, string> registerMap;        public:          IType(int op, string label);          IType(int op, string label, string rs, string rt, string immediate, map<string, string> registerMap);          virtual void print();          void execute(long registers[32], long dmem[32]);  };  

ITypeInstruction.cpp:

void IType::print()  {      cout << "Instruction Type: I " << endl;      cout << "Operation: " << label << endl;      cout << "Rs: " << registerMap[rs] << " (R" << rs << ")" << endl;      cout << "Rt: " << registerMap[rt] << " (R" << rt << ")" << endl;      cout << "Immediate: 0x" << immediate << endl;  }  

Decode.cpp:

Instruction *decode(string binaryIn, map<string, string> registerSet, map<string, string> instructionSet, Instruction *instructionPointer)  {          IType ins = IType(opcode, label, rs, rt, imm, registerSet);          Instruction i = IType(opcode, label, rs, rt, imm, registerSet);          ins.print();          *instructionPointer = ins;          (*instructionPointer).print();          return instructionPointer;  }  

main.cpp

Instruction *instructionToExecute;  instructionToExecute = decode(instructionToDecode, registerSet, instructionSet, instructionToExecute);  instructionToExecute->print();  

https://stackoverflow.com/questions/67014470/how-do-i-call-derived-functions-from-base-variables-in-c April 09, 2021 at 10:56AM