If a string's length is determined at compile-time, how can I properly initialize it?
#include <string>
int length = 3;
string word[length]; //invalid syntax, but doing `string word = " "` will work
word[0] = 'a';
word[1] = 'b';
word[2] = 'c';
...so that i can do something like this?
Example: http://ideone.com/FlniGm
My purpose for doing this is because I have a loop to copy characters from certain areas of another string into a new string.
A string is mutable and it's length can changed at run-time. But you can use the "fill constructor" if you must have a specified length: http://www.cplusplus.com/reference/string/string/string/
std::string s6 (10, 'x');
s6 now equals "xxxxxxxxxx".
You can initialize your string like this:
string word = "abc"
or
string word(length,' ');
word[0] = 'a';
word[1] = 'b';
word[2] = 'c';
a[0] = 'h'; a[1] = 'i'; instead of the more natural a = "hi";a += other_str;. The way you're accessing a[i] right now is wrong because there's no guarantee std::string has allocated up to the index you're trying to access. IOW assert(i < a.length());.
How about the following?
string word;
word.resize(3);
word[0] = 'a';
word[1] = 'b';
word[2] = 'c';
More on resizing a string: http://www.cplusplus.com/reference/string/string/resize/
word.reserve() instead of word.resize()? word.resize() without specifying another character puts null characters into the string, and these may have bad side effects later, if the application does not fill the string all the way up to the reserved size. word.reserve() reserves at least the provided minimum capacity, but doesn't change the string length and thus gives the application flexibility in filling it.You are probably looking for:
string word(3, ' ');
std::string does not support lengths known at compile time. There's even a proposal for adding compile time strings to the C++ standard.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4121.pdf
For now you're out of luck. What you can do is use static const char[] which does support compile time constant strings but obviously lacks some of the niceties of std::string. Whichever is appropriate depends on what you're doing. It may be that std::string features are unneeded and static char[] is the way to go or it may be that std::string is needed and the runtime cost is neglibible (very likely).
The syntax you are trying will work with static const char[]:
static const char myString[] = "hello";
Any constructor for std::string shown in the other answers is executed at runtime.
you can also simply initialize the the new string ans by
string ans= sourceString;
in this way you can copy rearranged string in ans.:)
std::stringwill figure out the rest. If you do mean compile time then no,std::stringdoesn't support this.