I am trying to create a function with an optional argument, where the optional argument gets modified/initialized in the function. This is part of a much larger program so I am using a header file. I am getting a compiler error and not sure how to fix it. Here is my code

Main

#include "header.h"
#include <iostream>
#include <stdlib.h>
#include <array>
#include <numbers>
#include <fstream>
#include <string>
#include <vector>
using namespace std;

int main(){
    int nr,np,nsize;
    nr = 3;
    nsize = nr  ;
    vector<double> b(nsize);
    vector<double> x(nsize);
    vector<vector<double>> A(nsize, vector<double>(nsize));
    func(nr,b,x,A,np);
    cout<< np <<endl;
}

Header

//same include statements, omitting for clarity
#ifndef HEADER_H
#define HEADER_H
     void func(int n,vector<double>& b,vector<double>& x,vector<vector<double>>& A,int np = -1);
#endif

func

#include "header.h"

void func(int n,vector<double> &b,vector<double> &x,
vector<vector<double>> &A,int &np){

    np =2;

}

I can get it to compile without the ampersand before np in func, but then the return value of np does not change in main. I also tried having the ampersand on np in the header file but that also gives me an error. Any advice would be appreciated.

Errors

Error with ampersand included in header.h:
header.h:24:97: error: cannot bind non-const lvalue reference of type 'int&' to an rvalue of type 'int'
24 | void gausspivot(int n,vector& b,vector& x,vector<vector>& A,int &np = -1)

Error with ampersand in func:
undefined reference to `func(int, std::vector<double, std::allocator >&, std::vector<double, std::allocator >&, std::vector<std::vector<double, std::allocator >, std::allocator<std::vector<double, std::allocator > > >&, int)' collect2.exe: error: ld returned 1 exit status

CodePudding user response：

You achieve that by function overloading, i.e. having the same function with different parameters. The function with less parameters calls the function with more parameters and provides the default value.

#include <iostream>
#include <vector>

void func(int n, std::vector<double>& b, std::vector<double>& x,
    std::vector<std::vector<double>>& A, int& np) {
    np = 2;
}

void func(int n, std::vector<double>& b, std::vector<double>& x,
    std::vector<std::vector<double>>& A) {
    std::cout << "You called the function with default value of -1\n";
    int dummy = -1;
    func(n, b, x, A, dummy);
    std::cout << "The value is now " << dummy <<"\n";
}

int main()
{
    std::vector<double> b;
    std::vector< std::vector<double>> A;
    func(0, b, b, A);
}

CodePudding user response：

One way to get this to compile is to have a global variable that serves as the default argument

#ifndef HEADER_H
#define HEADER_H

extern int np_default;

void func(int n, vector<double>& b,
     vector<double>& x, vector<vector<double>>& A,
     int& np = np_default);

#endif

Now this is ugly. Is the ugliness worth it? Only you can judge that.