尽管使用#ifndef，C++ 错误重新定义 function

Question

I've tried several times to get rid of the error, one of which was to use #ifndef but it hasn't worked yet. Please help!

vec3.h file

    #ifndef VEC3_H
#define VEC3_H

#include <cmath>
#include <iostream>

using std::sqrt;

class vec3 {
    public:
        vec3() : e{0,0,0} {}
        vec3(double e0, double e1, double e2) : e{e0, e1, e2} {}

        double x() const { return e[0]; }
        double y() const { return e[1]; }
        double z() const { return e[2]; }

        vec3 operator-() const { return vec3(-e[0], -e[1], -e[2]); }
        double operator[](int i) const { return e[i]; }
        double& operator[](int i) { return e[i]; }

        vec3& operator+=(const vec3 &v) {
            e[0] += v.e[0];
            e[1] += v.e[1];
            e[2] += v.e[2];
            return *this;
        }

        vec3& operator*=(const double t) {
            e[0] *= t;
            e[1] *= t;
            e[2] *= t;
            return *this;
        }

        vec3& operator/=(const double t) {
            return *this *= 1/t;
        }

        double length() const {
            return sqrt(length_squared());
        }

        double length_squared() const {
            return e[0]*e[0] + e[1]*e[1] + e[2]*e[2];
        }

    public:
        double e[3];
};

// Type aliases for vec3
using point3 = vec3;   // 3D point
using color = vec3;    // RGB color

#endif
  
  
    // vec3 Utility Functions

inline std::ostream& operator<<(std::ostream &out, const vec3 &v) {
    return out << v.e[0] << ' ' << v.e[1] << ' ' << v.e[2];
}

inline vec3 operator+(const vec3 &u, const vec3 &v) {
    return vec3(u.e[0] + v.e[0], u.e[1] + v.e[1], u.e[2] + v.e[2]);
}

inline vec3 operator-(const vec3 &u, const vec3 &v) {
    return vec3(u.e[0] - v.e[0], u.e[1] - v.e[1], u.e[2] - v.e[2]);
}

inline vec3 operator*(const vec3 &u, const vec3 &v) {
    return vec3(u.e[0] * v.e[0], u.e[1] * v.e[1], u.e[2] * v.e[2]);
}

inline vec3 operator*(double t, const vec3 &v) {
    return vec3(t*v.e[0], t*v.e[1], t*v.e[2]);
}

inline vec3 operator*(const vec3 &v, double t) {
    return t * v;
}

inline vec3 operator/(vec3 v, double t) {
    return (1/t) * v;
}

inline double dot(const vec3 &u, const vec3 &v) {
    return u.e[0] * v.e[0]
         + u.e[1] * v.e[1]
         + u.e[2] * v.e[2];
}

inline vec3 cross(const vec3 &u, const vec3 &v) {
    return vec3(u.e[1] * v.e[2] - u.e[2] * v.e[1],
                u.e[2] * v.e[0] - u.e[0] * v.e[2],
                u.e[0] * v.e[1] - u.e[1] * v.e[0]);
}

inline vec3 unit_vector(vec3 v) {
    return v / v.length();
}

main file, chap_3.cpp

    #include "color.h"
#include "ray.h"
#include "vec3.h"

#include <iostream>


color ray_color(const ray& r) {
    vec3 unit_direction = unit_vector(r.direction());
    auto t = 0.5*(unit_direction.y() + 1.0);
    return (1.0-t)*color(1.0, 1.0, 1.0) + t*color(0.5, 0.7, 1.0);
}

int main() {

    // Image

    const auto aspect_ratio = 16.0 / 9.0;
    const int image_width = 400;
    const int image_height = static_cast<int>(image_width / aspect_ratio);

    // Camera

    auto viewport_height = 2.0;
    auto viewport_width = aspect_ratio * viewport_height;
    auto focal_length = 1.0;

    auto origin = point3(0, 0, 0);
    auto horizontal = vec3(viewport_width, 0, 0);
    auto vertical = vec3(0, viewport_height, 0);
    auto lower_left_corner = origin - horizontal/2 - vertical/2 - vec3(0, 0, focal_length);

    // Render

    std::cout << "P3\n" << image_width << " " << image_height << "\n255\n";

    for (int j = image_height-1; j >= 0; --j) {
        std::cerr << "\rScanlines remaining: " << j << ' ' << std::flush;
        for (int i = 0; i < image_width; ++i) {

            auto u = double(i) / (image_width-1);
            auto v = double(j) / (image_height-1);
            ray r(origin, lower_left_corner + u*horizontal + v*vertical - origin);
            color pixel_color = ray_color(r);

            write_color(std::cout, pixel_color);
        }
    }

    std::cerr << "\nDone.\n";
}

How can I solve errors like the following:

enter codevec3.h: In function 'vec3 operator+(const vec3&, const vec3&)':
vec3.h:65:17: error: redefinition of 'vec3 operator+(const vec3&, const vec3&)'
 inline vec3 operator+(const vec3 &u, const vec3 &v) {
             ^~~~~~~~
In file included from color.h:4:0,
             from chap_3.cpp:1:
vec3.h:65:17: note: 'vec3 operator+(const vec3&, const vec3&)' previously defined here
     inline vec3 operator+(const vec3 &u, const vec3 &v) {
                 ^~~~~~~~ here

I've tried several sources but to no avail, also I picked the code from a trusted source and have made only minor changes so I am not able to understand why the error is occuring.

Answer 1

Your inline definitions are after the #ifndef / #endif surrounding the rest of the header file, meaning they will be picked up by every #include . You can either

1. Move the #endif to the very end of the header file, or
2. Use #pragma once instead of the include guards (not standard but most compilers support it).