如何在标头和 .cpp 文件中正确声明返回自己定义类型的函数？

Question

I am new to working with headers and the whole abstractization thing in C++ and I have noticed that: when you declare a function in the .cpp file, you first specify the return-type of the function, followed by namespace_name :: function_signature , the name and the arguments of the function being the same you have already specified in the .h file. Yet, how should I write the signature of a function whose forward-declaration ( if that is how the signatures in the .h file are called ) is present in the same namespace as the own-defined return type? ( sorry for the verbose, code is simpler than my question; see below for clarification )

I want to find the longest sequence of equal numbers in a given array. These are my files:

//"sequences.h" file
#ifndef LAB1_SEQUENCES_H
#define LAB1_SEQUENCES_H

namespace Sequences{

/**
 * Abstract Data Type for a sequence
 */
struct seq{
    int i;
    int j;
};

/**
 * Finds longest sequence of equal integers
 * arr - array to search in
 * n - number of elements in the array
 */
seq longest_equal(int* arr, int n);

}
#endif //LAB1_SEQUENCES_H

.

//"sequences.cpp" file
#include "sequences.h"


//Sequences :: seq Sequences :: longest_equal(int *arr, int n)
//seq Sequences :: longest_equal(int *arr, int n)
//struct seq longest_equal(int *arr, int n)
//Sequences :: struct seq longest_equal(int *arr, int n)
{
int i = 0, j = i + 1;
int lmax = -1, iR = -1, jR =-1 ; // lmax ar trebui sa fie 1 sau -1 ?
while (j < n )
{
    if (arr[i] == arr[j]) j ++;
    else
    {
        if (j - i > lmax) { lmax = j - i; iR = i; jR = j; }
        i = j;
        j = i + 1;
    }
}
if (lmax > 1) return Sequences :: seq(iR, jR);
return Sequences::seq(-1, -1);
}

Now, I do not know how I can make my longest_equal function in the .cpp file return seq type (I am regarding both the signature in the header file and the signature in the .cpp file, as well as what I should virtually write before the return commands at the end of the function's implementation, in the .cpp file).

I have tried the commented lines of code just before the first accolade in the second file as signatures of the function. I have also read the answers to the question here and some answers of the question here and tried something myself, yet to no avail.

Answer 1

Forward declarations are used to reduce header dependencies. Since your struct seq doesn't require any headers and all it has is data members, it makes little sense to forward declare it. You should define it in the header along with the function declaration (and have the function definition in the .cpp ).

You can use a forward declaration only for taking and returning structures and classes by reference or pointer. In your particular case the struct seq is a simple aggregate value, a tuple with named fields, conceptually. It is not abstract data type as you state because there is nothing abstract about it, it is rather concrete and have no behaviour. An abstract data type is defined in terms of an behaviour (functions and member functions), rather than data members it is composed of.