No visible overloaded '=' in C++ vector

Question

I'm trying to assign a value to a vector, but I get this error. What is the issue about?

std::vector<bool> *boolVec = new std::vector<bool>(max, true);
boolVec[0] = false;

This code works:

boolVec->at(0) = false;

`ptr[index]` is equivalent with `*(ptr + index)`, so `boolVec[0]` is the same as `*boolVec`. You are trying to assign a `bool` to the vector itself. — The Paramagnetic Croissant, Nov 24 '15 at 21:27
I'm voting to close this question as off-topic because it is trivially resolvable by paying attention. — The Paramagnetic Croissant, Nov 24 '15 at 21:27
Are you sure you even want to `new` the vector? `std::vector boolVec(max, true);` probably does what you want. — user2357112 supports Monica, Nov 24 '15 at 21:28
The advantages of not using `new` are explained [here](http://stackoverflow.com/q/6500313/509868) — anatolyg, Nov 24 '15 at 21:33
Dont add the answer to the question. If you think doing `boolVec->at` is the answer then add it as an answer, but know that `->at(` is not equivalent to `operator[]`. — Fantastic Mr Fox, Nov 24 '15 at 21:38

score 4 · Answer 1 · answered Nov 24 '15 at 21:28

4

try:

(*boolVec)[0] = false;

You have a pointer to a vector, so you have to dereference the pointer first.

answered Nov 24 '15 at 21:28

Adam

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Fantastic Mr Fox · Accepted Answer · 2015-11-24T21:41:17.330

3

You have a pointer to a vector std::vector<bool> *boolVec This is not what you want. Just use a vector:

std::vector<bool> boolVec(max, true);
boolVec[0] = false;

The vector data will be stored on the heap.

If you truely need the pointer then you need to dereference it. You can use

(*boolVec)[0] = false;

or

 boolVec->at(0) = false;

Note that using ->at( will perform boundary checks and may be slower.

edited Nov 24 '15 at 21:41

answered Nov 24 '15 at 21:28

Fantastic Mr Fox

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This initializes vector on stack, I may have very large vector size, thus I want to create it on heap. I also don't want to just push the value to vector, but set a specific value at index. – Dvole Nov 24 '15 at 21:29
6

@Dvole: The part that holds the bools goes on the heap either way. – user2357112 supports Monica Nov 24 '15 at 21:31
@Dvole The heavy part will be on the heap. This is absolutley what you want, dont make a pointer to a vector. – Fantastic Mr Fox Nov 24 '15 at 21:31
@Ben Thats something I always wanted to know but never dared to ask ;). – 463035818_is_not_a_number Nov 24 '15 at 21:32
@user2357112 thanks for clearing this up. – Dvole Nov 24 '15 at 21:33
There certainly are reasons to have a pointer to a vector, but the stated one is not one of them. – Benjamin Lindley Nov 24 '15 at 21:33
@BenjaminLindley Id love an example of a reason. Only thing i can think of is passing over physical interfaces or something crazy like that. Can you give me an example? – Fantastic Mr Fox Nov 24 '15 at 21:35
@Ben: Same reason you'd want a pointer to anything else. As a non-owning reassignable/nullable reference. – Benjamin Lindley Nov 24 '15 at 21:36
@BenjaminLindley, while I totally agree one might need a pointer to vector, I really have hard time trying to come with a valid scenario of dynamically creating a vector object. Just see no reason to. – SergeyA Nov 24 '15 at 22:09
@SergeyA: I'm only talking about the pointer part, not the dynamic allocating part. Although I can certainly also think of scenarios where I would want to dynamically allocate a vector, it would never be of the form `vector<...>* vp = new vector<...>;`. It might come in the form of `std::vector> vv;`. I would, in fact, never use `new` for anything (other than placement new), even when I want dynamically allocated raw memory. – Benjamin Lindley Nov 24 '15 at 22:19
@BenjaminLindley, interesting approach. How would you design a container of polymorphic types? – SergeyA Nov 24 '15 at 22:29
@SergeyA: Okay, you got me. I was using hyperbole when I said "never". I'll use it when it is the most convenient interface (for your example, it would be a container of `unique_ptr`, which would result in using `new` before `std::make_unique` existed). But if `unique_ptr` didn't already exist, I would not implement my version of it using allocating new. It would just use `malloc` with placement new. – Benjamin Lindley Nov 24 '15 at 22:40

Vlad from Moscow · Answer 3 · 2015-11-24T21:52:41.383

There are several ways to use the subscript operator

For example

( *boolVec )[0] = false;

or

boolVec[0][0] = false;

or

boolVec->operator []( 0 ) = false;

Here is a demonstrative program

#include <iostream>
#include <iomanip>
#include <vector>


int main( void )
{
    std::vector<bool> *boolVec = new std::vector<bool>( 6, true );

    boolVec[0][0] = false;
    ( *boolVec )[2] = false;
    boolVec->operator []( 4 ) = false;

    for ( bool b : *boolVec ) std::cout << std::boolalpha << b << ' ';
    std::cout << std::endl;

    delete boolVec;
}


false true false true false true

Consider also a possibility to define tha vector like

std::vector<bool> boolVec( 6, true );

No visible overloaded '=' in C++ vector

3 Answers3