Hi,

So I received some code and a memo saying, I quote, "problems with overflows". I check it out, and I see something along the lines of the following:

char cmdstr[20];
sprintf_s(cmdstr,"STAS %d,%d\n",min(m_fstart,m_fstart2),max(m_fstop,m_fstop2));
viPrintf(vi,cmdstr);
viPrintf(vi,"CHAN1\n");
// ---SNIP---

Immediately, I changed it to this:

std::stringstream cmdstr;
cmdstr << "STAS " << min(m_fstart,m_fstart2) << "," << max(m_fstop,m_fstop2) << std::endl;
viPrintf(vi,cmdstr.str().c_str()); // viPrintf is not const-correct, i.e. cmdstr is declared as "char*", not "const char*"

HOWEVER, as stated in the comments above, the function "viPrintf" is not const correct. Even though it only reads form "cmdstr", the function argument was declared as "char*" and the compiler will refuse to compile it.

I cannot change viPrintf because it is part of an external library I don't have access to.

How can I go about converting a string stream to a non-const char array? The thought of copying it into a new std::string object and using &str.at(0) crossed my mind, but that seems a little... I don't know, is there a better way?

An additional question, is it dangerous to do that if viPrintf() were to change the char array you provide it with? cmdstr.str().c_str() is const because its not meant to be written to.

Alright, thank you very much! Didn't know about const_cast until now.

If the function turns out to modify, don't fall back on manually allocating the memory, just use a vector:

std::string s = whatever();

std::vector<char> v(s.begin(), s.end());
viPrintf(v.data());

Then you still have exception safety and so on if viPrintf throws something.

[EDIT] Hmm, actually, that probably doesn't copy the null character but you get the idea...

Thanks for the help! The solution I went with was:

std::stringstream cmdstr;
cmdstr << "STAS " << min(m_fstart,m_fstart2) << "," << max(m_fstop,m_fstop2) << std::endl;
viPrintf(vi,const_cast<char*>(cmdstr.str().c_str()));

It turns out viPrintf() really doesn't modify the data, it only reads from it.

Another way of potentially doing it to avoid a const cast would be:

viPrintf(vi, &cmdstr.str().at(0));

Obviously you'd need to null-terminate the stream before you do that:

cmdstr << '\0';

I think const_cast is cleaner and less error prone.