Strcat, 2 char arrays of pointers

Question

I have such function, which get 2 arrays as char arguments, i need to merge this strings. Sorry for easy question.

    void Abc (char **a, char **b)
{

    for (int i = 0; i < 200; i++)
    {
        a[i]="hello ";
    }

    for (int i = 0; i < 200; i++)
    {
        strcat(a[i], b[i]);       //doesn't show mistake, but it's there
        cout << b[i]<<'\n';        //ok
        cout << a[i]<<'\n';        //ok
        cout<<strlen(b[i])<<'\n';  //ok
        cout<<strlen(a[i])<<'\n';  //ok
    }
}

Why Strcat doesn't work here? What can i do to make this work?

Answer 1

Here's the source of your problem:

for (int i = 0; i < 200; i++)
{
    a[i]="hello ";

The compiler doesn't know your intent here, so every element of a[] is assigned a pointer to the same string

a[0] == a[1] == a[2] == a[3] == a[4] == ... a[199]

Even if this worked, the problem is that all elements of a now point to a char array of size 7 {'h', 'e', 'l', 'l', 'o', 0}. You can't stract onto the end of them.

You need to ensure that each element of a[] has a pointer to a unique storage space and that it is large enough to receive your strings. If your a[] elements are already pointing somewhere valid, what you need to do is:

for (int i = 0; i < 200; i++)
{
    strcpy(a[i], "hello ");
}

Answer 2

First, every a[i] is pointing to a string literal and modifying it invokes undefined behavior.

Second, when you're strcat(x, y) you should make sure that x has enough space to store merged x and y.

Therefore, x should has allocated memory and to double check, you shoud use strncat to avoid unintended overflow.

Answer 3

The problem is in the following statement.

a[i]="hello ";

Instead of copying the string literal you assign its address.

Use instead

strcpy( a[i],"hello " );