Create a default constructor in C++

c++ default-constructor
user1824239 picture user1824239 · Nov 14, 2012 · Viewed 34.7k times · Source

This might be a stupid question but I can't find a lot of information on the web about creating your own default constructors in C++. It seems to just be a constructor with no parameters. However, I tried to create my default constructor like this:

Tree::Tree()  {root = NULL;}

I also tried just: 

Tree::Tree() {}

When I try either of these I am getting the error:

No instance of overloaded function "Tree::Tree" matches the specified type.

I can't seem to figure out what this means.

I am creating this constructor in my .cpp file. Should I be doing something in my header (.h) file as well?

Answer

Pete Becker picture Pete Becker · Nov 14, 2012

Member functions (and that includes constructors and destructors) have to be declared in the class definition:

class Tree {
public:
    Tree(); // default constructor
private:
    Node *root;

};

Then you can define it in your .cpp file:

Tree::Tree() : root(nullptr) {
}

I threw in the nullptr for C++11. If you don't have C++11, use root(0).

Related questions

no default constructor exists for class

#include "Includes.h" enum BlowfishAlgorithm { ECB, CBC, CFB64, OFB64, }; class Blowfish { public: struct bf_key_st { unsigned long P[18]; unsigned long S[1024]; }; Blowfish(BlowfishAlgorithm algorithm); void Dispose(); void SetKey(unsigned char data[]); unsigned char Encrypt(unsigned char buffer[]); unsigned char …

Read More
Does the default constructor initialize built-in types?

Does the default constructor (created by the compiler) initialize built-in-types?

Read More
C++ default destructor

When I don't declare a constructor for example, the compiler will provide me with a default constructor that will have no arguments and no definition (body), and thus, will take no action. If I now don't declare a destructor, the …

Read More