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Convert from std::string to unsigned char*

Im very much a newbie but perhaps somehone can help me. Ive been
searching for a way to convert a std::string to a unsigned char*

The situation is I have a function that wants a unsigned char* and I
want to give it a std::string

no matching function for call to `MD5::update(st d::string&, size_t)'
candidates are: void MD5::update(uns igned char*, unsigned int)

void PrintMD5(string str){
MD5 context;

context.update( str, str.size());
context.finaliz e();

cout << "MD5: " << context << endl;


I hope someone can help me solve this delema.

Aug 21 '06
10 26670

"Howard" <al*****@hotmai l.comwrote in message
news:kV******** ************@bg tnsc04-news.ops.worldn et.att.net...
"Jim Langston" <ta*******@rock etmail.comwrote in message
news:7E******** *****@newsfe02. lga...
>std::string.c_ str() will return a const char *. Notice very carefully
this is const. That means that the contents can't be changed through

If the
context.update ( unsigned char*, unsigned int )
is going to change the contents of the char *, which I highly expect it
will, you can't use this.

My solution in these cases is to actually copy the c_str() to a char
array to copy it back.

unsigned char* TempString = new unsigned char[ str.size() * 2 ];
strcpy( TempString, str.c_str() );
context.update ( TempString, strlen( TempString ) );

It's a bad idea to simply "guess" what a function will do with a string,
and how long an array you might need. Such guesses are bound to either 1)
fail miserably at some point, or 2) use up memory needlessly. And
nobody's going to understand why you've chosen to arbitrarily double the
I agree. I had planned on explaining why I was using str.size() * 2 instead
of simply str.size() but had forgot. My bad.
I doubt that update() is updating the string. More likely, it's updating
the "context" object. If it _is_ updating the string, then it should be
doing the allocation as well. It's poorly designed if you tell it how
much data it _can_ use, but it decides how much it _needs_ to use. What
should it do if it needs more than you gave it?
I doubt it highly too. Most likely the method is just not const
correctness, and if the OP has the source to the class he should fix it to
be const unsigned char*.

I use a library where I don't have the source (it's a .dll) but only the
interface so can't mess with the signatures by fixing the const correctness
myself and in those cases I use a const_cast because I know for a fact that
the function is not going to change the string.

Aug 21 '06 #11

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