Member Avatar for Dazaa

I am sure there is nothing wrong with this... I am racking my brain as to why it wont compile.

#include <iostream>
#include <string>

using namespace std;

class Vehicle {
	public:
		Vehicle();
		Vehicle(string m, int y);
		void print() const;
	protected:
		string model;
		int year;
};

void Vehicle::print() const {
	cout << "Model = " << model << " , year = " << year << endl;
}

int main() {
	Vehicle a; //something extremely basic is moaning about this line
                   //if i use Vehicle a(); it seems to work but the rest of it has                                //problems because i used the parenthesis
	a.print();	
}

the error is:

C:/Documents and Settings/Daryl/My Documents/159234/study1/inheritance.cpp:22: undefined reference to `Vehicle::Vehicle()'

Edited by Dazaa because: n/a
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Dani AI

Generated

As already hinted: the linker error "undefined reference to Vehicle::Vehicle()" means you declared a default constructor inside the class but never provided its definition. The compiler accepted the class declaration, but the linker could not find the constructor body to produce an object.

Provide a definition (and initialize members) or remove the need for a default constructor. For example, define a safe default constructor that initializes model and year:

Vehicle::Vehicle() : model("Unknown"), year(0) {}

Or, in C++11 and later, explicitly request the compiler-generated default constructor if that matches your intent:

Vehicle() = default;

If you intended to put the constructor body in a separate .cpp file, make sure that file is compiled and linked with the rest of the program (for example: g++ -std=c++11 main.cpp vehicle.cpp -o program). A common cause of "undefined reference" is forgetting to link the translation unit that contains the definition — see this explanation on linker errors (Stack Overflow).

One more pitfall to note (this explains why Vehicle a(); "seemed to work"): Vehicle a(); is not an object construction — it is a function declaration (the "most vexing parse"). Use Vehicle a; to create a default object, or supply parameters: Vehicle a("Model", 2010);. For details see the C++ references on default constructors and the most vexing parse: default constructor and .

Summary checklist:

  • Define the declared Vehicle() or remove/replace it.
  • Initialize built-in members (like int year) to avoid garbage.
  • If the definition is in another file, compile/link that file.

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Member Avatar for Lerner

It sounds like your compiler wants you to define the default constructor since you declared it and called it. The easiest response to this would be to add a set of curly brackets after the declaration of the default constructor and rebuild/run the program again.

Note that your program doesn't have the default constructor give the member variables any meaningful default values so calling print() object a will give junk for output, though it shouldn't stop compilation.

Edited by Lerner because: n/a
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