How assign std::string through a function?

Question

So, How can i assign std::string through a function?

i have this code

void WP_Hash(std::string dest, std::string src) { // Set up all the locals struct NESSIEstruct whirlpool_ctx; char digest[64], buffer[4100]; unsigned char *tmp = (unsigned char *)digest; // Start the hashing NESSIEinit(&whirlpool_ctx); NESSIEadd((const unsigned char *)src.c_str(), src.length() * 8, &whirlpool_ctx); // Finish the hashing NESSIEfinalize(&whirlpool_ctx, tmp); // Convert to a real string unsigned int len = 0; while (len< 128) { buffer[len++] = gch[*tmp >> 4]; buffer[len++] = gch[*tmp & 0x0F]; tmp++; } buffer[128] = 0; dest.assign(buffer); }

and this code to initialize it:

 std::string ret; WP_Hash(ret, "none"); sampgdk::logprintf("%s", ret.c_str());

It print nothing

when i change ret string to "a", it print "a" i want it print none (hashed in WP_Hash but ignore about it, let's say that "none" is the result of WP_Hash)

what could i do?

Lightness Races in Orbit · Accepted Answer · 2015-06-07 01:24:39Z

C++ is not Java: its objects have value semantics. So, you're passing a copy into that function.

Pass a reference instead if you want the original argument to keep the changes from within the function. (It is also worth passing a (const) reference to src to save copying it unnecessarily.)

 void WP_Hash(std::string& dest, const std::string& src); // ^ ^^^^^^ ^

Alternatively, return the result string from the function instead:

 std::string WP_Hash(const std::string& src); // ^^^^^^^^^^^ ^^^^^^ ^

then use as such:

const std::string ret = WP_Hash("none"); sampgdk::logprintf("%s", ret.c_str());

MSalters · Accepted Answer · 2015-06-07 01:20:40Z

0

Same as any other C++ type - pass by value unless specified otherwise. Add a & to make it a reference. Or just return it.

answered Jun 7, 2015 at 1:20

MSalters

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4 Comments

Raefaldhi Amartya Over a year ago

can you giveme example where i can put & ?, i have try to put it WP_Hash(&ret, "none"); but it show error Cannot convert argument 1 from 'std::string *' to 'std::string'

Lightness Races in Orbit Over a year ago

@RaefaldhiAmartya: Programming by guessing will not work. You should put some effort into actually learning the language.

Raefaldhi Amartya Over a year ago

@LightnessRacesinOrbit ok thanks for the suggestion. i dont have a c++ book, i just read c++ tutorial in tutorialspoint, i dont find what i search about.

Lightness Races in Orbit Over a year ago

@RaefaldhiAmartya: Get a C++ book. You cannot learn the language from random tutorials on the internet. In fact, you're more likely to learn it wrong!

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