问：

我有以下简单的结构：

struct Wire {
    int start_x;
    int end_x;
    int start_y;
    int end_y;

    Wire(int sx, int ex, int sy, int ey): start_x(sx), end_x(ex), start_y(sy), end_y(ey) {}
};

我还有以下打印函数，可以从第一项中获取并打印出一些值：std::vector<Wire>

这些函数是相同的，除了一个按值取参数，另一个按引用取参数。

void print_first_wire_value(std::vector<Wire> wires){
    std::cout << "(" << wires[0].start_x << ", " << wires[0].start_y << ")" << std::endl;
}

void print_first_wire_reference(std::vector<Wire> &wires){
    std::cout << "(" << wires[0].start_x << ", " << wires[0].start_y << ")" << std::endl;
}

我的主函数包括以下代码。

它使用另一个函数用连线对象填充向量。然后，它调用这两个打印函数。

std::vector<Wire> wires = input_to_wires(first_line);

std::cout << "By value: " << std::endl;
print_first_wire_value(wires);
std::cout << "By reference: " << std::endl;
print_first_wire_reference(wires);

我知道第一个start_x和end_x值是 0 和 0。两个打印函数都应打印这些值。但奇怪的是，我反而得到了这个：

By value: 
(-543694264, -543694264)
By reference: 
(0, 0)

这到底是怎么回事？

最小可重现示例

（请原谅这些看似奇怪的名字，这是解决代码出现后编程挑战的开始）。

day3_lib.h

#pragma once
#include <vector>
#include <string>

struct Wire{
    Wire(int sx, int ex, int sy, int ey);
};

std::vector<Wire> populate_wires();

void print_first_wire_value(std::vector<Wire> wires);

void print_first_wire_reference(std::vector<Wire> &wires);

day3_lib.cpp

#include <vector>
#include <sstream>
#include <iostream>
#include <stdexcept>

struct Wire {
    int start_x;
    int end_x;
    int start_y;
    int end_y;

    Wire(int sx, int ex, int sy, int ey): start_x(sx), end_x(ex), start_y(sy), end_y(ey) {}
};

std::vector<Wire> populate_wires(){
    std::vector<Wire> wires;

    Wire wire1(0, 0, 1003, 0);
    Wire wire2(1003, 0, 1003, -138);
    Wire wire3(1003, 0, 662, -138);

    wires.push_back(wire1);
    wires.push_back(wire2);
    wires.push_back(wire3);

    return wires;
}

void print_first_wire_value(std::vector<Wire> wires){
    std::cout << "(" << wires[0].start_x << ", " << wires[0].start_y << ")" << std::endl;
}

void print_first_wire_reference(std::vector<Wire> &wires){
    std::cout << "(" << wires[0].start_x << ", " << wires[0].start_y << ")" << std::endl;
}

第3天.cpp

#include "./day3_lib.h"
#include <fstream>
#include <iostream>

int main(){
    std::vector<Wire> wires = populate_wires();

    std::cout << "By value: " << std::endl;
    print_first_wire_value(wires);
    std::cout << "By reference: " << std::endl;
    print_first_wire_reference(wires);
}