How to change a vector item in C++?

Question

I've got a vector of structs in C++ and I would like to modify each item individually. I found that doing SomeStruct info = myVector[i] gives me a copy of the item, so if I modify it nothing will be changed. So right now I'm resetting the item like that: myVector[i] = info. Is there a more efficient way do that? One that won't involve a copy operation?

This is my current code:

struct CharacterInfo {
    QChar character;
    int occurrences;
    double frequency;
};

std::vector<CharacterInfo> characterInfos;

// Some code to populate the vector

for (unsigned i = 0; i < characterInfos.size(); i++) {
    CharacterInfo info = characterInfos[i];
    info.frequency = (double)info.occurrences / (double)totalOccurrences;
    characterInfos[i] = info; // how to avoid this?
}

Answer 1

The simplest way which doesn't change too much of your code is just to use a reference instead of an instance. So:

SomeStruct & info = myVector[i];

The next easiest way is to change from using a loop with an index, so like:

for (std::vector<SomeStruct>::iterator it = myVector.begin(); it != myVector.end(); ++it)
{
    SomeStruct & info = *it;
    // do stuff here
}

With the STL you can go even further, especially if you have a C++11 capable compiler, for instance:

std::for_each(std::begin(myVector), std::end(myVector), [](SomeStruct & info) { /* do stuff here */ });

Also not related to your question directly, but if you add a method to the struct that computes the frequency, the code becomes much cleaner, for instance following from the last example you could do:

std::for_each(std::begin(myVector), std::end(myVector), std::mem_fun(&SomeStruct::calculateFrequency));

This will also work without a C++11 compiler if you change the calls to std::begin(myVector) with myVector.begin() and the same for end.

Answer 2

You can use a reference:

CharacterInfo& info = characterInfos[i];
info.frequency = (double)info.occurrences / (double)totalOccurrences;

The reference info is bound to the element of your vector. If you change it, you change the vector element too.