Concatenation of chars-CodePudding

I need to concatenate to chars and send it as argument to function, but strcat move concatenated chars to first char. I need a method that return concatenated char. Or if I can do it in different way how can I do it?

void abca(char *a)
{
  Serial.println(a);
}


void setup()
{
  Serial.begin(9600);

  char *bb = "1";
  char *aa = "2";
  abca(strcat(aa, bb));

}

Edit: I'm creating program for Arduino, and I can't use strings. Strings use a lot of memory. Unfortunately Arduino have only 2kB

CodePudding user response：

There are a few problems with your code. First of all, you are using non-const pointers to const arrays:

char *bb = "1"; // "1" is a char constant array
char *aa = "2"; // so it needs a pointer to constant memory

When you fix that your function calls will fail because they need non-const arrays.

One fix is to create a non-const array to receive your concatenated string:

// should be const because it is not modified
void abca(char const* a)
{
    Serial.println(a);
}

void setup()
{
    Serial.begin(9600);

    // char const array decays to pointer to const char
    char const* bb = "1";
    char const* aa = "2";

    // can't concatenate into constant memory
    // abca(strcat(aa, bb));

    // So make some writable memory and concat to that
    char buffer[32]; // long eough for the combined text

    strcpy(buffer, bb);
    strcat(buffer, aa);

    abca(buffer);
}

If you don't know how big the resulting string is you may need to allocate buffer dynamically to make sure it is big enough.

CodePudding user response：

If you are able to change the function to

void abca(const char *a)

Then you could use a std::string at the calling site and the c_str() method:

int main()
{
    std::string a = "aa";
    std::string b = "bb";
    abca((a   b).c_str());
}

Note that the overloaded operator is used for concatenation. Better still, if you can change the function to

void abca(const std::string& a)

(note that the body of the function is unchanged), you can write

abca(a   b);

at the calling site. All standard C , and no memory leaks or undefined behaviour either! std::string might get some bad press but it does epitomise the power of C .