C++输出未定义 ++q

Question

请问这个输出未定义是什么意思？
std::cout << square(++q) << ' ' << q << std::endl; //输出未定义，++q被执行了两次 结果为9 3;

#include <iostream>
#define sum(x, y) (x) + (y)    //#define 本质上是文本替换  如下第五行中2 * sum(3, 5)  运行后为2 * 3 + 5 = 11；
#define square(x) ((x) * (x))
int main() {
    std::cout << sum(1, 2) << std::endl << 2 * sum(3, 5) << std::endl;
    int q = 1;
    std::cout << square(++q) << ' ' << q << std::endl;   //输出未定义，++q被执行了两次 结果为9 3;
    std::cout << std::endl;
    std::cout << std::endl;
    int i = 1, a = 0, b = 0;
    a = i++;      //先赋值后自增   a = i, i = i + 1;
    b = ++i;      //先自增后赋值   i = i + 1, b = i;
    printf("i = %d, a = %d, b = %d",i,a,b);
    return 0;     //结果：i = 3, a = 1, b = 3
}

Answer 1

std::cout << square(++q) << ' ' << q << std::endl; 
展开变成
std::cout <<( (++q)*(++q)) << ' ' << q << std::endl;