CODEWITHSUNDEEP
c++stringinsertresizecapacity
anon_user
anon_user

Reputation: 43

C++ How to preserve string capacity when copying

EDIT (original post in edit history)

I'm able to reproduce my problem with this example:

#include <string>
#include <vector>
using namespace std;

#define MAX_BUFFER 30

int main(int argc, char **argv) {

    vector<string> myVec = { "hey","asd","haha" };
    vector<string> clone;

    for (int i = myVec.size(); i--;) {
        myVec[i].reserve(myVec[i].size() + MAX_BUFFER);
        clone.push_back(myVec[i]);
    }

    return 0;
}

Add a breakpoint before return 0; . Now, inspect strings that are in myVec and clone. Their capacity is not the same!

Upvotes: 0

Views: 475

Answers (3)

songyuanyao
songyuanyao

Reputation: 172864

The capacity is not requested to be copied to be same when std::string being copied.

$21.3.1.2/2 basic_string constructors and assignment operators [string.cons]:

Table 49 — basic_string(const basic_string&) effects
Element   Value
data()    points at the first element of an allocated copy of the array whose first element is pointed at by str.data()
size()    str.size()
capacity()    a value at least as large as size()

The only guarantee is the capacity of the string will be at least as large as its size after copying.

It means you have to do this by yourself:

for (int i = myVec.size(); i--;) {
    myVec[i].reserve(myVec[i].size() + MAX_BUFFER);
    clone.push_back(myVec[i]);
    clone.back().reserve(myVec[i].capacity());
}

Upvotes: 4

twobit
twobit

Reputation: 61

myString.resize(l) will not change the string's capacity, except when l is large then the string's size. Just call myString.capacity() and see for yourself.

Upvotes: 0

Zeta
Zeta

Reputation: 981

Yes, string class has a member function String resize member funtion..

Upvotes: 0

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