說明

在STL中 vector和string 是比較特殊的，clear()之后是不會釋放內存空間的，也就是size()會清零，但capacity()不會改變，需要手動去釋放，說明 clear() 沒有釋放內存。
想釋放空間的話，除了swap一個空string外，c++11里新加入的的std::basic_string::shrink_to_fit?也可以。

代碼

注意string的swap清空方法為：string().swap(str);

vector的swap清空方法為：nums.swap(vector<int>());

#include <iostream> #include <string>int main() {std::string s;std::cout << "Default-constructed capacity is " << s.capacity()<< " and size is " << s.size() << '\n';for (int i = 0; i < 42; i++)s.append(" 42 ");std::cout << "Capacity after a couple of appends is " << s.capacity()<< " and size is " << s.size() << '\n';s.clear();std::cout << "Capacity after clear() is " << s.capacity()<< " and size is " << s.size() << '\n';s.shrink_to_fit();std::cout << "Capacity after shrink_to_fit() is " << s.capacity()<< " and size is " << s.size() << '\n';for (int i = 0; i < 42; i++)s.append(" 42 ");std::cout << "Capacity after a couple of appends is " << s.capacity()<< " and size is " << s.size() << '\n';string().swap(s);std::cout << "Capacity after swap() is " << s.capacity()<< " and size is " << s.size() << '\n'; }

輸出為：

Default-constructed capacity is 15 and size is 0 Capacity after a couple of appends is 235 and size is 168 Capacity after clear() is 235 and size is 0 Capacity after shrink_to_fit() is 15 and size is 0 Capacity after a couple of appends is 235 and size is 168 Capacity after swap() is 15 and size is 0

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