return value of a bool function and passed pointer as parameter

Question

I am new to C++ having trouble in assigning value to char* of a function. I have a function as below which returns bool

bool Function(char* inString)
{
        int m = strlen(inString);
    char output[1001];
    memset(output , 0 , sizeof(output));
    sprintf_s(output,50,"length is %d",m);

       if(m>5)
        return true;
    if(m<5) 
        return false;

}

Along with the function , i am trying to get the "output" value on calling this function outside defined local inside this function which has value - "length is -"

I tried doing

 bool Function(char* inString)
{
int m = strlen(inString);
    char output[1001];
    memset(output , 0 , sizeof(output));
    sprintf_s(output,50,"length is %d",m);
    sprintf_s(inString,50,output);
  if(m>5)
            return true;
        if(m<5) 
            return false;
}

But this fails because inString has already a value and this is giving following error Access violation writing location 0x00165267.

Is there any way to get both parameters from this function ( bool value based on string length) as well as b) the string statement "output"?

I appreciate your help..

And if `m==5`? Your functions doesn't cover returns for all possibilities. — Alexandru Barbarosie, Jul 02 '13 at 08:50
m==5 is file-not-found obviously: http://thedailywtf.com/Articles/What_Is_Truth_0x3f_.aspx — Bernd Elkemann, Jul 02 '13 at 08:51

Joe Z · Accepted Answer · 2013-07-02T09:27:13.707

How are you calling this function? If you're calling it with a string literal, then that string literal is likely in read-only memory. For example, this will fail:

bool result = Function( "longer than 5" );

That will likely result in an access violation because the string "longer than 5" is likely in read-only memory. (It's not required to be, but it's likely with modern compilers.)

Also, as Alexandru pointed out above, you didn't handle the m==5 case at all.

Edit: If you want to get the string you're generating with sprintf_s out of the function, you have a couple options.

You could add a second parameter with a buffer to print the string into. Instead of declaring a buffer on the stack (your char output[1001]), you could instead have this pointer passed to you. ie. your function prototype becomes bool Function(char *inString, char *outString);
Make your buffer static, and then return a pointer to it by reference. That requires you to add a second operand. This one is a bit uglier: bool Function(char *inString, char **outString); Then, inside your code you say *outString = output, which is, frankly, gross.
Move your output buffer to a file-scope array. That's even uglier.

One of the advantages of #1 above is that you can use default arguments and an if-statement to make the sprintf optional:

bool Function(char *inString, char *outString = 0)
{
    int m = strlen(inString);

    if (outString)
        sprintf_s(outString, 50, "length is %d", m);

    return m >= 5;
}

Also, a stylistic note: Prefer char *inString to char* inString. In C and C++, the asterisk is not part of the type being declared, but rather a modifier on the individual symbol being declared. If you want to declare two character pointers a and b, you write char *a, *b, not char* a, b.

Edit 2: If you believe John Carmack's insistence that you const anything that could be const, then the above becomes:

bool Function(const char *const inString, char *const outString = 0)
{
    const int m = strlen(inString);

    if (outString)
        sprintf_s(outString, 50, "length is %d", m);

    return m >= 5;
}

I suppose you could go further and make it return const bool, except the function's return value is already an rvalue...

Yeah, most compilers would catch and optimize that. I'll go ahead and edit my answer above, though. — Joe Z, Jul 02 '13 at 09:26
If you ever find a compiler that *doesn't* "optimize" that, it's time to throw that compiler away. But Roddy's suggestion isn't a performance optimization, it's a *readability* optimization. — Cody Gray - on strike, Jul 02 '13 at 09:28
I just find it more readable. Unless you happen to prefer `if ((m>=5) == true) ...` ;-) — Roddy, Jul 02 '13 at 09:28
@Roddy and @Cody Gray: I agree with you that the more concise form is more readable. I've worked with too many programmers, though, that need the longer, more-spelled-out form, sadly. They get confused when they see you returning a comparison as a value, as if comparisons are only valid inside an `if`, `while` or `for`. This isn't Pascal, folks. — Joe Z, Jul 02 '13 at 09:33
Awesome! Thats impressive answer. I was exactly looking for this solution. Thanks Joe and everyone! — junni lomo, Jul 02 '13 at 09:35

score 1 · Answer 2 · edited Nov 27 '13 at 16:15

To be able to return a string from the function. You need the string to be allocated in such a way that it is alive even after you exit the function. Your example stores the string in a local array(output), which does not live beyond the scope({ }) of the function.

There are several ways to do this:

Dynamically allocated memory
static storage

and so on, which one to use depends on your usage semantics.

You just need to pass the output string as a function parameter. Since you want to allocate the string inside the function you need to pass a pointer by reference.

On a side note, consider using std::string instead of a char *

score 0 · Answer 3 · edited May 23 '17 at 11:45

Since you are using c++ you should do something like

bool Function(std::string &inString, std::string &outString)
{
    size_t size = inString.size();
    outString = "length is " + std::to_string(size); //if c++11
    if (size > 5)
            return true;
    return false;
}

to convert your size_t to string in C++ you can use boost::lexical_cast or this define

#include <sstream>
#define SSTR( x ) dynamic_cast< std::ostringstream & >(( std::ostringstream() << std::dec << x ) ).str()

 inString = "length is " + SSTR(size);

score 0 · Answer 4 · answered Jul 02 '13 at 09:24

Your code is not really C++, it's C.

Here's how you should do it in C++. Note the use of references rather than pointers.

bool Function(const std::string &inString, std::string &outString)
{
  int m = inString.size();

  std::stringstream ss;
  ss << "length is" << m;

  outString = ss.str();

  return m >= 5;
}

return value of a bool function and passed pointer as parameter

4 Answers4