函数声明的顺序在 Visual Studio C++ 2019（Unity 版本）中是否重要？

Question

I am learning C++ coming from a Java/C#/Lua/Python background.

I decided to learn the language by writing some small games using the OpenGL package (I'm also writing some text parsers and re-writing a scripting language I wrote in C#, in C++ to the same ends). I have the following test code I am using to set up my project:

// OpenGLExample01.cpp : This file contains the 'main' function. Program execution begins and ends there.
#include <iostream>
#ifdef __APPLE_CC__
    #include <GLUT/glut.h>
#else
    #include <GL/glut.h>
#endif

extern "C"
{
    #include <lua.h>
    #include <lualib.h>
    #include <lauxlib.h>
}

// Initializes GLUT, the display mode, and main window; registers callbacks;
// enters the main event loop.
int main(int argc, char** argv) {

    // Use a single buffered window in RGB mode (as opposed to a double-buffered
    // window or color-index mode).
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);

    // Position window at (80,80)-(480,380) and give it a title.
    glutInitWindowPosition(80, 80);
    glutInitWindowSize(400, 300);
    glutCreateWindow("A Simple Triangle");

    // Tell GLUT that whenever the main window needs to be repainted that it
    // should call the function display().
    glutDisplayFunc(display);

    // Tell GLUT to start reading and processing events.  This function
    // never returns; the program only exits when the user closes the main
    // window or kills the process.
    glutMainLoop();
}

static void display() {

    // Set every pixel in the frame buffer to the current clear color.
    glClear(GL_COLOR_BUFFER_BIT);

    // Drawing is done by specifying a sequence of vertices.  The way these
    // vertices are connected (or not connected) depends on the argument to
    // glBegin.  GL_POLYGON constructs a filled polygon.
    glBegin(GL_POLYGON);
    glColor3f(1, 0, 0); glVertex3f(-0.6, -0.75, 0.5);
    glColor3f(0, 1, 0); glVertex3f(0.6, -0.75, 0);
    glColor3f(0, 0, 1); glVertex3f(0, 0.75, 0);
    glEnd();

    // Flush drawing command buffer to make drawing happen as soon as possible.
    glFlush();
}

This code won't compile when I try to build it. The reason? In glutDisplayFunc(display), "display" is undefined. However, if I flip the functions around (ie define display() first, then define main()), the program builds without a hitch.

#include <iostream>
#ifdef __APPLE_CC__
    #include <GLUT/glut.h>
#else
    #include <GL/glut.h>
#endif

extern "C"
{
    #include <lua.h>
    #include <lualib.h>
    #include <lauxlib.h>
}

static void display() {

    // Set every pixel in the frame buffer to the current clear color.
    glClear(GL_COLOR_BUFFER_BIT);

    // Drawing is done by specifying a sequence of vertices.  The way these
    // vertices are connected (or not connected) depends on the argument to
    // glBegin.  GL_POLYGON constructs a filled polygon.
    glBegin(GL_POLYGON);
    glColor3f(1, 0, 0); glVertex3f(-0.6, -0.75, 0.5);
    glColor3f(0, 1, 0); glVertex3f(0.6, -0.75, 0);
    glColor3f(0, 0, 1); glVertex3f(0, 0.75, 0);
    glEnd();

    // Flush drawing command buffer to make drawing happen as soon as possible.
    glFlush();
}

// Initializes GLUT, the display mode, and main window; registers callbacks;
// enters the main event loop.
int main(int argc, char** argv) {

    // Use a single buffered window in RGB mode (as opposed to a double-buffered
    // window or color-index mode).
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);

    // Position window at (80,80)-(480,380) and give it a title.
    glutInitWindowPosition(80, 80);
    glutInitWindowSize(400, 300);
    glutCreateWindow("A Simple Triangle");

    // Tell GLUT that whenever the main window needs to be repainted that it
    // should call the function display().
    glutDisplayFunc(display);

    // Tell GLUT to start reading and processing events.  This function
    // never returns; the program only exits when the user closes the main
    // window or kills the process.
    glutMainLoop();
}

Here's the issue: Everything I've read thus far suggests that the first code snippet should be functionally the same as the second snippet. What's going on here?

Answer 1

yes, in order to use it, the compiler need to see the declaration.

Only the declaration is needed though, not necessary the definition.

static void bar(); // declaration

void foo(){
    bar();
}

static void bar(){ // function body here (definition)
  /*do something*/
};