CODEWITHSUNDEEP
c++constructor
Mike1982
Mike1982

Reputation: 459

creating a c++ constructor with parameters

I'm refreshing how to write constructors in C++, but the C++ documentation version is throwing silly compiler errors.

I've looked at other examples from stack overflow and mimicked them and still similar errors

assume #includes are already here .h file

class Matrix{
    private:
    //eventually going to create 2d vector here
    public:
      // from: http://www.cplusplus.com/doc/tutorial/classes/
      Matrix (string);
      //another way shown from other examples
      Matrix (string filename); 
  };

.cpp

Matrix::Matrix(string filename){
    int num = 0;
    string line;
    ifstream matrix_file(filename);
    if(matrix_file.is_open()){
      while(getline(matrix_file, line)){
        stringstream extract(line);
        while(extract >> num){
          cout << num << " ";
        }
        cout << '\n';
      }
      matrix_file.close();
    }

  }

main.cpp

int main(int argc, char *argv[]){

    string filename = argv[1];
    Matrix grid (filename);
    return 0;
  }

I'm expecting upon object creation when the constructor is called, it prints out the values in the file. But when compiling, I get:

matrix.h:6:12: warning: unnecessary parentheses in declaration of ‘string’ [-Wparentheses]
     Matrix (string);
            ^
matrix.h:6:19: error: field ‘string’ has incomplete type ‘Matrix’
     Matrix (string);
                   ^
matrix.h:2:7: note: definition of ‘class Matrix’ is not complete until the closing brace
 class Matrix{

or

matrix.h:6:19: error: expected ‘)’ before ‘filename’
     Matrix (string filename);
            ~      ^~~~~~~~~
                   )
main.cpp: In function ‘int main(int, char**)’:
main.cpp:11:24: error: no matching function for call to ‘Matrix::Matrix(std::__cxx11::string&)’
   Matrix grid (filename);

depending on which way I initialize the string parameter in .h file. I'm thinking I have a small typo somewhere, but I'm not finding anything wrong with this simple piece of code. Any help would be much appreciated. Thanks

Upvotes: 0

Views: 1199

Answers (2)

henhen
henhen

Reputation: 1203

you need to include <string> and scope it with std:: since string is also part of the std namespace. You can do using namespace std for convenience. For constructor definition in header, i think it’s cleaner to omit the parameter name and just have the type.

Upvotes: 2

jignatius
jignatius

Reputation: 6484

There are a few problems with your code.

Firstly, you haven't included <string>. You will also need to include <sstream>, <fstream> and <iostream>. It's not considered good practice to include a whole namespace with using namespace std. So prefix types from the standard library with std:: - i.e. std::string, std::ifstream, std::stringstream and std::cout.

Secondly, you have duplicate constructors taking one std::string parameter:

Matrix(string);

Matrix(string filename);

You will get a compiler error for the second line saying the member function is already defined or declared. That's because you're trying to define the same constructor twice. It is optional to use parameter names for functions in the header file. However, for readability it is best to have them there. So I would just go with:

Matrix(std::string filename);

You could change this line to accept a const std::string& in order to avoid copying. However, that's up to you.

Upvotes: 2

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