How to return a char* from a function in C++

Question

I have the following function in c++

    char* Test::convertToSHA(const char* cc) {

    const char* salt ="sh$^$#@!&7hbfvatfacv@@@@bagg=shjgvshvcbschj";
    time_t currentTime;
    time(&currentTime);
    CCString startTimeString;
    startTimeString.createWithFormat("%d", currentTime);

    std::string s = cc;
    s += startTimeString.getCString();
    s += salt;

    char *str = new char[strlen(s.c_str()) +  1];
    int length=strlen(str);
    unsigned char hash[length*2];
    char hexstring[41];
    sha1::calc(str,length,hash);
    sha1::toHexString(hash, hexstring);

    return hexstring;
}

And in the call i use

char* output=NULL;
output= Test::convertToSHA("hello");

This is causing my code to crash. Is there a problem with me returning a string ? How can i return from this function ?

Do NOT return stack variables. Either `new hexstring`, or let the caller to allocate memory and pass it to `convertToSHA`. But the better way is to return std::string (or return by reference) since you're using c++. — Mine, Dec 26 '13 at 06:26
Why are you creating (and forgetting to populate) a temporary buffer, `str`? You could just write `sha1::calc(s.data(), s.length(), hash);`. — Marcelo Cantos, Dec 26 '13 at 06:44

score 5 · Answer 1 · edited May 23 '17 at 11:56

5

You can never return a local pointer in C or C++. Instead, either pass the pointer as a parameter:

void Test::convertToSHA(const char* cc, const char* hexstring) {

   //no change to body
   //no return statement needed
}

//then when calling the function:

char hexstring[41];

convertToSHA(cc, hexstring);

//hexstring has been modified

Or just use std::string instead. Use the c_str() if you need to convert to a c-style string (you still can't return the pointer, though).

edited May 23 '17 at 11:56

Community

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answered Dec 26 '13 at 06:31

ApproachingDarknessFish

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I have modified the code. Its still crashing. – AndroidDev Dec 26 '13 at 06:36
1

@BadLuckBrian Show us the new code. – ApproachingDarknessFish Dec 26 '13 at 06:36

score 3 · Accepted Answer · answered Dec 26 '13 at 06:34

I'm assuming you're using this library. I find it odd that it claims to be written in C++ but doesn't take advantage of C++ idioms. The function modifies the pointer, and doesn't return anything, so you do need a local variable. Lucky for you, std::strings will happily accept a char [] argument.

std::string Test::convertToSHA(const char* cc) {
    // ...
    char hexstring[41];
    sha1::calc(str,length,hash);
    sha1::toHexString(hash, hexstring);
    std::string ret(hexstring);

    return ret;
}

Yes thanks. I do use that Library. Its working now. Thanks for your help. — AndroidDev, Dec 26 '13 at 06:41

How to return a char* from a function in C++

2 Answers2