Creating a vector of char pointers in C++

Question

I have to make a vector that stores char pointers. Each pointer should only point to a single char like 'a' or 'b'. I don't get any compile errors, but when I run my program, it crashes. I cannot use std::string or any of its functions, they have to just be characters.

std::vector<char*> myVector;
myVector.reserve(10);
myVector.at(0) = new char['a'];

Why does the program crash? Shouldn't I be able to create new char pointers in memory as I go?

`myVector.reserve` -> `myVector.resize` https://en.cppreference.com/w/cpp/container/vector/resize — Richard Critten, Feb 25 '21 at 19:21
Also, `new char['a']` doesn't do what you think it does (you probably want `new char('a')`). — Stephen Newell, Feb 25 '21 at 19:22
A pointer to a single char seems odd and wasteful. Why not just a vector of char? — Retired Ninja, Feb 25 '21 at 19:23
Why not remove the headaches of character arrays and use `std::string` and `std::vector>`? — Thomas Matthews, Feb 25 '21 at 19:25
`char chars[] = "ab"; std::vector myVector(10); myVector.at(0) = &chars[0];`? — MikeCAT, Feb 25 '21 at 19:25
@StephenNewell, I switched to parentheses and still the same error. The error also does not even tell me what the problem is, which adds to my problem. — The_Redhawk, Feb 25 '21 at 19:35
I changed from reserve to resize, and that got rid of the crash, but my requirements say specifically to use reserve which is unfortunate. — The_Redhawk, Feb 25 '21 at 19:38
Then use `resize()` after `reserve()` or use [`push_back()`](https://en.cppreference.com/w/cpp/container/vector/push_back) to add element(s). — MikeCAT, Feb 25 '21 at 19:39
`at(0)` will throw an exception, you could track the exception source in the debugger — M.M, Feb 25 '21 at 19:46

Ho1 · Accepted Answer · 2021-02-25T19:47:15.300

3

You should use push_back method of vector instead of using at when you have not pushed anything inside vector. Also, you should use new char('a') instead of using bracket, which has other meaning. Look at this minimal code that works just fine:

#include <iostream>
#include <vector>

int main()
{
    std::vector<char*> myVector;
    myVector.push_back(new char('a'));
    std::cout << *myVector[0] << std::endl;
}

edited Feb 25 '21 at 19:47

answered Feb 25 '21 at 19:42

Ho1

1,239
1
11
29

2

You won't need `static_cast()` because `*myVector[0]` is already a `char`. – MikeCAT Feb 25 '21 at 19:45
@MikeCAT You're right. i was playing with the code to find the problem. – Ho1 Feb 25 '21 at 19:47

score 1 · Answer 2 · answered Feb 25 '21 at 19:39

Let's see line by line:

std::vector<char*> myVector;

Is there a need to keep a vector of char pointers?
What's following tells us you only want to store 1 char in there, so just use std::string, it'll be way easier.

myVector.reserve(10);

The reserve() methods are only used for pre-allocation, they don't actually change the size of your vector, only its capacity. For every vector: 0 <= size <= capacity. std::vector::at() will simply throw if you try to access something outside [0;size[

myVector.at(0) = new char['a'];

As we said, the .at(0) call will throw an exception here. But something funny will happen on the right side. Since allocating a new array is usually done with new T[N] where T is the type of the elements and N the size of the array, this will implicitly convert the char 'a' into an int (which is 97) and therefore allocate an array of 97 uninitialized elements.

So I adjusted to this ``` myVector.reserve(10); myVector.push_back(new char('a')); ``` The error is now gone and I can print 'a' to the screen using std::cout << *myVector.at(0); Without the * to dereference the vector element, it prints jibberish to my screen. This means that I am dealing with a pointer to a char correct? — The_Redhawk, Feb 25 '21 at 19:44

Creating a vector of char pointers in C++

2 Answers2