写c++程序时，g++出现错误提示undefined reference to

我把类的声明和定义，全部放在main函数所在的代码里了，但编译过程中还是出现undefined reference to （类的成员），查了各种方法，搞了一晚上，已经崩溃了。有大佬知道是什么问题吗。
cpp source:

using namespace std;
#include "ope_on_StackPair.h"
StackPair::StackPair() {
    for (int & i : a)i = 0;
    stack1.end = a;
    stack2.end = a + NUM - 1;
    stack1.length = stack2.length = 0;
}


struct stack StackPair::pop(struct stack sta) {
    if (sta.length > 0)
        sta.length -= 1;
    return sta;
}

struct stack StackPair::push(struct stack sta , int con) {
    if (stack1.length + stack2.length < NUM) {
        if (sta.end == a) {
            a[NUM - sta.length - 1] = con;
        } else a[sta.length] = con;
        sta.length += 1;
    } else std::cout << "There's no spare space." << std::endl;
    return sta;
}

int StackPair::inspect(struct stack sta) {
    if (sta.length == 0) {
        std::cout << "The stack is void. The returned is not true." << std::endl;
        return 999999;
    }
    else {
        if (sta.end == a)return a[sta.length - 1];
        else return a[NUM - sta.length];
    }
}

int StackPair::check(struct stack sta) {
    if (sta.length == 0)return 1;
    else return 0;
}

cpp header

using namespace std;
#include<iostream>
#define NUM 20  //The maximum length of the stack pair can be changed here, with default value of 20.
struct stack{          //'struct stack' defines the structure of the stack pair.
    int* end;
    int length;
};

class StackPair {
public:
    static struct stack pop(struct stack sta);

    static struct stack push(struct stack sta , int con);

    static int inspect(struct stack sta);

    static int check(struct stack);

    StackPair();

    static int a[NUM];
    static struct stack stack1, stack2;
};

cmakelist:

cmake_minimum_required(VERSION 3.20)
project(stack_pair)

set(CMAKE_CXX_STANDARD 14)

add_executable(stack_pair main.cpp)
add_library(ope_on_StackPair ope_on_StackPair/ope_on_StackPair.cpp)
target_link_libraries(stack_pair ope_on_StackPair)

代码呢

确认代码没错的话可以在终端里执行类似命令

g++ test.cpp -o test

李奶奶的，光定义了一个类，忘记定义对象了，已解决，谢谢