String.length woes - c++

Question

Edit: Solutions must compile against Microsoft Visual Studio 2012.
I want to use a known string length to declare another string of the same length.
The reasoning is the second string will act as a container for operation done to the first string which must be non volatile with regards to it.
e.g.
const string messy "a bunch of letters";
string dostuff(string sentence) {
string organised NNN????? // Idk, just needs the same size.
for ( x = 0; x < NNN?; x++) {
organised[x] = sentence[x]++; // Doesn't matter what this does.
}
}
In both cases above, the declaration and the exit condition, the NNN? stands for the length of 'messy'.
How do I discover the length at compile time?

Answer 1

std::string has two constructors which could fit your purposes.
The first, a copy constructor:
string organised(sentence);
The second, a constructor which takes a character and a count. You could initialize a string with a temporary character.
string organised(sentence.length(), '_');
Alternatively, you can:
Use an empty string and append (+=) text to it as you go along, or
Use a std::stringstream for the same purpose.
the stringstream will likely be more efficient.
Overall, I would prefer the copy constructor if the length is known.

Answer 2

std::string isn't a compile time type (it can't be a constexpr), so you can't use it directly to determine the length at compile time.
You could initialize a constexpr char[] and then use sizeof on that:
constexpr char messychar[] = "a bunch of letters";
// - 1 to avoid including NUL terminator which std::string doesn't care about
constexpr size_t messylen = sizeof(messychar) / sizeof(messychar[0]) - 1;
const string messy(messychar);
and use that, but frankly, that's pretty ugly; the length would be compile time, but organized would need to use the count and char constructor that would still be performed on each call, allocating and initializing only to have the contents replaced in the loop.
While it's not compile time, you'd avoid that initialization cost by just using reserve and += to build the new string, which with the #define could be done in an ugly but likely efficient way as:
constexpr char messychar[] = "a bunch of letters";
constexpr size_t messylen = sizeof(messychar) / sizeof(messychar[0]) - 1;
// messy itself may not be needed, but if it is, it's initialized optimally
// by using the compile time calculated length, so there is no need to scan for
// NUL terminators, and it can reserve the necessary space in the initial alloc
const string messy(messychar, messylen);
string dostuff(string sentence) {
string organised;
organized.reserve(messylen);
for (size_t x = 0; x < messylen; x++) {
organised += sentence[x]++; // Doesn't matter what this does.
}
}
This avoids setting organised's values more than once, allocating more than once (well, possibly twice if initial construction performs it) per call, and only performs a single read/write pass of sentence, no full read followed by read/write or the like. It also makes the loop constraint a compile time value, so the compiler has the opportunity to unroll the loop (though there is no guarantee of this, and even if it happens, it may not be helpful).
Also note: In your example, you mutate sentence, but it's accepted by value, so you're mutating the local copy, not the caller copy. If mutation of the caller value is required, accept it by reference, and if mutation is not required, accept by const reference to avoid a copy on every call (I understand the example code was filler, just mentioning this).