C++ header file error (LNK1120: 1 unresolved externals)

Question

I try to make simple example from simplest book on C++. But I receive a error. Please, be lenient to my poor knowledge. This is not duplicate of C++ LNK1120 Fatal Error: 1 unresolved externals

File prime.h

bool is_prime(int);

File prime.cpp

// prime.cpp
#include <cmath> // Needed for sqrt
#include "prime.h" // is_prime prototype
/** is_prime(n)
* Determines the primality of a given value
* n an integer to test for primality
* Returns true if n is prime; otherwise, returns false
*/
bool is_prime(int n) {
    bool result = true; // Provisionally, n is prime
    double r = n, root = sqrt(r);
    // Try all possible factors from 2 to the square
    // root of n
    for (int trial_factor = 2; result && trial_factor <= root; trial_factor++)
        result = (n % trial_factor != 0);
    return result;
}

File for tests.cpp

#include <iostream>
#include "prime.h"
/*
* main
* Tests for primality each integer from 2
* up to a value provided by the user.
* If an integer is prime, it prints it;
* otherwise, the number is not printed.
*/
int main() {
    int max_value;
    std::cout << "Display primes up to what value? ";
    std::cin >> max_value;
    for (int value = 2; value <= max_value; value++)
        if (is_prime(value)) // See if value is prime
            std::cout << value << " "; // Display the prime number
    std::cout << '\n'; // Move cursor down to next line
}

They are in 1 folder:

prime.h is included as header

Subsystem Console is chosen:

I get such error. Please help me.

------ Build started: Project: for tests, Configuration: Release x64 ------

for tests.obj : error LNK2001: unresolved external symbol "bool __cdecl is_prime(int)" (?is_prime@@YA_NH@Z)

D:_c++\for tests\x64\Release\for tests.exe : fatal error LNK1120: 1 unresolved externals

Done building project "for tests.vcxproj" -- FAILED.

========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Possible duplicate of [What is an undefined reference/unresolved external symbol error and how do I fix it?](https://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix) — Alan Birtles, Oct 12 '19 at 06:12
@AlanBirtles It lies in the same folder with other files. This is not enough? How do I carry out your advice? — Igor K., Oct 12 '19 at 06:16
Just add it to your project the same as you have for the header files — Alan Birtles, Oct 12 '19 at 06:18
what source files are in the source files folder? I just tried to manually compile your code with the cl compiler directly, it works fine. — Josh Weinstein, Oct 12 '19 at 06:51

C++ header file error (LNK1120: 1 unresolved externals)

0 Answers0