strcat alternative issue C++

Question

I am working on a project which was initially sampled in C but want to work on it in C++.

There is a section where a strcat() is used, I have been told to use an alternative. I have found one here, but when I try those the compiler gives me the following error:

error: invalid operands of types char*' andchar*' to binary `operator+'

Is there something I am doing wrong?

Edit:

Here's the portion of the code that doesn't work

FILE *FileOpen(string *fname, string* mode){
FILE *fp;
string *str = "";

str += "tmp/"; //all files to be created in temporary sub directory
str += fname;
if((fp=fopen(str,mode))==NULL){
fprintf(stderr,"Cannot open file: %s\n", &fname);
exit(1);
}
FileReader(fname);
return(fp);
}

Edit 2: For those wondering why I have FileReader: it's for part 2 of the project. Disassembling a code.

Answer 1

If your code is using char * as strings, then strcat is probably the right function for you. Of course, the C++ solution is to use std::string, in which case you can just use + - since there is binary operator+ available for std::string.

Answer 2

You haven't shown any code, but I suspect you had something like this

char *s1 = "Hello, ", *s2 = "world!";
char buf[50];
strcpy(buf, s1);
strcat(buf, s2);

and now you changed it to

char *s1 = "Hello, ", *s2 = "world!";
char buf[50];
buf = s1 + s2;

This doesn't work, as you already noticed. You must change the char pointers and char array to std::string as well

std::string s1 = "Hello, ", s2 = "world!";
std::string buf = s1 + s2;

Answer 3

Thank you for posting your code; now the problem is readily apparent.

You should use string objects, not pointers to them.

FILE *FileOpen(string fname, string mode)
{
    string str = "";

    str += "tmp/"; //all files to be created in temporary sub directory
    str += fname;
    FILE *fp = fopen(str.c_str(), mode.c_str());
    if (!fp) {
        fprintf(stderr, "Cannot open file: %s\n", fname.c_str());
        exit(1);
    }
    FileReader(fname);
    return fp;
}

A good next step would be to move to I/O functions that accept std::string arguments, so you don't have to say .c_str() everywhere.

I'm also confused why you have FileReader(fname) inside your file-opening function. That violates the Single-Responsibility-Prinicple, twice. Opening a file should not cause it to be read, and the code reading the file should use a FILE* and not care what the filename is (except perhaps for generation of error messages).

Answer 4

well, C++ has a string class std::string whose operator+ performs concatenation. But you have to create one first.

so the expression"abc" + "def" doesn't compile, but std::string("abc")+"def" works fine.

alternatively you can write something like

std::string s("abc");
s += "def";

similarly,

std::string s = "abc";
s += "def";

if you want to concatenate a large amount of text, and care about the performance, consider using std::ostringstream.