Comparison between signed and unsigned integer exp

问题:

I just started using OpenGL. This is my first code:

// OpenGL hello program
#include<iostream>
#include <GL/glut.h>
#include <cstring>

void display() {
    glClear(GL_COLOR_BUFFER_BIT);
    char message[] = "Hello, world!";
    glRasterPos2d(0, 0);
    for (int i = 0; i < sizeof(message) / sizeof(message[0]); i++)
    {
        glutBitmapCharacter(GLUT_BITMAP_HELVETICA_12, message[i]);
    }
}

int main(int argc, char *argv[]) {
    glutInit(&argc, argv);
    glutInitWindowSize(500, 500);
    glutCreateWindow("OpenGL hello program");
    glutDisplayFunc(display);
    glutMainLoop();
}

The error I am getting is: Warning: comparison between signed and unsigned integer expressions (line 9). I also tried writing a new code then to see whats causing the problem:

#include<iostream>
#include <cstring>

void display1() {
    char message[] = "Hello, world!";

    for (int i = 0; i < sizeof(message) / sizeof(message[0]); i++)
        std::cout<<message[i];
}

int main() {
    display1();
}

This code works perfectly fine. Why is the first code not working fine?

EDIT: Following up on Cyber's annswer, I changed the loop to:

for (unsigned int i = 0; i < sizeof(message) / sizeof(message[0]); i++)

But the OpenGL code does not do the expected i.e. show "Hello, world!" message in the window. It just creates a window with "OpenGL hello program" written at the top and nothing else.

回答1:

This line is the problem

for (int i = 0; i < sizeof(message) / sizeof(message[0]); i++)

The operator sizeof has the return type of std::size_t which is therefore what you should use for the type of your variable i. std::size_t is an unsigned type, so the compiler is warning you that you are comparing a signed type (int) to an unsigned type, because the comparison is potentially unsafe if the value of one variable is not in the representable range of in the other type.

for (std::size_t i = 0; i < sizeof(message) / sizeof(message[0]); ++i)

Or simply use a range-based for loop.

for (auto i : message)
{
    std::cout << i;    // i is a char from the message array
}