为什么即使在第一步中已经vectors[0]
,我仍然仍然可以从pop_back()
中获取值?我猜现在我的vectors
是空的,并通过调用empty()
#include <iostream>
#include <vector>
struct Vector2 {
int x, y;
Vector2(int _x, int _y)
: x(_x)
, y(_y) {}
Vector2(const Vector2& vec)
: x(vec.x)
, y(vec.y) {
std::cout << "[Copy] Vector2{" << x << "," << y << "} copied!"
<< std::endl;
}
void Print() {
std::cout << "[Print] Vector2{" << x << "," << y << "}" << std::endl;
}
};
void PrintVectors(const std::vector<Vector2>& vectors) {
std::cout << "---" << std::endl;
for (auto& vector : vectors) {
std::cout << "Vector2{" << vector.x << "," << vector.y << "}"
<< std::endl;
}
}
int main() {
std::vector<Vector2> vectors;
vectors.push_back({21, 78});
PrintVectors(vectors);
// First
vectors.pop_back();
if (vectors.empty()) {
std::cout << "Empty condition!" << std::endl;
}
vectors[0].Print(); // ---> I thought I'd get an error at this point.
vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
PrintVectors(vectors);
// Second
vectors.pop_back();
vectors[0].Print();
vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
PrintVectors(vectors);
// Third + no pop_back needed
vectors[0].Print();
vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
PrintVectors(vectors);
// 4th
vectors.pop_back();
vectors[0].Print();
vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
PrintVectors(vectors);
}
然后,这种情况使我可以通过gdb进行调试。我看到_M_
的值在变化,但是我不知道发生了什么(我看不到_M_start
和_M_finish
之间的模式或关系)。