如何用长度初始化std :: string？

Question

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.

Answer 1

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" .

Answer 2

You can initialize your string like this:

string word = "abc"

or

string word(length,' ');
word[0] = 'a';
word[1] = 'b';
word[2] = 'c';

Answer 3

您可能正在寻找：

string word(3, ' ');

Answer 4

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.

Answer 5

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/