C ++，擦除向量

Question

i'm new in cpp.I defined a vector with this code and in the end , i want to erase it completely.

#include <string>
#include <stdio.h>
#include <vector>
#include <iostream>
using namespace std;


struct MCLASS
{
    string *a = new string();
    string *b = new string();

    MCLASS(string ax,string bx)
    {
        a = new string(ax);
        b = new string(bx);
    }

    ~MCLASS()
    {
        delete a;
        delete b;
    }
};

vector<MCLASS*> *vc = new vector<MCLASS*>;




int main()
{
    for (int i = 0 ; i <= 1000000 ; i++)
        vc->push_back(new MCLASS("EHEM","UHUM"));

    for (int i = 0 ; i <= 1000000 ; i++)
        delete vc->at(i);

    vc->clear();
    delete vc;
    cout<<"Deleted";
    cin.get();

    return 0;
}

but there is a problem.I can't erase it completely.I tried to delete each items of vector , clear vector items , and delete it.but when i check the RAM Usage of this application , it uses about 130MB.Please explain to me what to do.Thanks!

Answer 1

You're calling new string twice per member variable. For each one, the result of the second call to new string overwrites the result of the first, so the first never gets delete d and there's your memory leak.

Don't call new string in the declaration of a and b —only in the constructor.

struct MCLASS
{
    string *a;
    string *b;
...

Answer 2

From cplusplus.com's article on vector::clear :

A reallocation is not guaranteed to happen, and the vector capacity is not guaranteed to change due to calling this function. A typical alternative that forces a reallocation is to use swap: vector<T>().swap(x); // clear x reallocating vector<T>().swap(x); // clear x reallocating

As VladimirS suggested in his comment; memory management is not a simple beast, however the above should reduce your memory consumption provided nothing has prevented that by replacing delete (such as some memory inspection tools).

Answer 3

If you want to erase all contents of a std::vector you can either call the clear member function or resize it to zero size.

There's a slight difference in result. clear will destroy all elements but (likely) preserve the vectors capacity. resize(0) will destroy all elements and also reduce capacity to zero.

Answer 4

Your error takes place on lines 10-11 :

string *a = new string();
string *b = new string();

Indeed, you create two pointers a and b and you allocate memory for them, memory you'll never free because you re-allocate new memory in the constructor without checking values.

Proper way to fix that (and good practice) is to initialize pointers attributes to NULL . In your case replace these lines like that:

string *a = NULL;
string *b = NULL;

then, allocate it in constructor or somewhere...

Tips :

• Before a new or a delete , check current values ;)
• Use Valgrind or a tool like that to check memory leak