Initialize vector passed by reference

Question

I have a class Pixel and a class Image with a function used to update a pixel line. I want to initialize the pixel line. My problem is to initialize the vector. Actually I have this :

bool UpdateLine(std::vector<Pixel>& line, int nb)
{
    bool noError = true;
    line = new std::vector<Pixel>(nb);
    // Do some stuff
    return noError;
}

When I try this I have :

no viable overloaded '='

How can I initialize the vector ?

in your code, you are not *initializing* the vector, but *assigning* to it. — JFMR, Sep 26 '17 at 09:48
Please familiarize yourself with the language first. I recommend reading at least one or two [books about C++](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — AMA, Sep 26 '17 at 09:51
@tobi303 everything *is* initialized, possibly to indeterminate values for default-initialized built-in types. — Quentin, Sep 26 '17 at 09:55

JFMR · Accepted Answer · 2017-09-26T10:02:20.490

3

The expression you are using:

new std::vector<Pixel>(nb)

returns a pointer to std::vector<Pixel> (i.e.: std::vector<Pixel> *), and not a std::vector<Pixel>, which is what you need.

A distinction between initialization and assignment has to be done: you are assigning to line and not initializing it.

You can create a new vector and assign it to line by means of an assignment operator:

line = std::vector<Pixel>(nb);

edited Sep 26 '17 at 10:02

answered Sep 26 '17 at 09:53

JFMR

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Thank you, I will take my book again, I probably misunderstood – A.Pissicat Sep 26 '17 at 10:02

gsamaras · Answer 2 · 2017-09-26T09:58:47.510

What you do is assignment, not initialization.

Initializing a vector could be done like this:

std::vector<int> second (4,100); // four ints with value 100

In your case however, the vector is already declared outside of the function, so in that case you use vector::resize and do this:

line.resize(nb); // creates 'nb' objects 'Pixel'

PS: You don't need new keyword, except if you dynamically allocating memory for your vector.

Initialize vector passed by reference

2 Answers2