[转贴]char s[]字串和char *s字串有什么差別?
C語言有兩種字串宣告方式char s[]和char *s,兩者有什麼差異呢?
Introduction
char s[] = "Hello World";
char *s = "Hello World";
char *s = "Hello World";
皆宣告了s字串,在C-style string的函數皆可使用,但兩者背後意義卻不相同。
char s[] = "Hello World";
的s是個char array,含12個byte(包含結尾\0),"Hello World"對s來說是initializer,將字元一個一個地copy進s陣列。
char *s = "Hello World";
的s是一個pointer指向char,由於"Hello World"本身就是一個string literal,所以s指向"Hello World"這個string literal的起始記憶體位置。
做個簡單的實驗證明兩者不同
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#include <iostream>
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using namespace std;
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int main() {
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char s1[] = "Hello World";
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char *s2 = "Hello World";
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cout << "size of s1: " << sizeof(s1) << endl;
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cout << "size of s2: " << sizeof(s2) << endl;
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}
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執行結果
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s1是陣列,所以占了12 byte,而s2只是pointer,所以占了4 byte,實驗結果與預期相同。
實際使用有什麼不同嗎?兩種寫法皆可使用substring和pointer寫法,但只有char *s可以直接使用*s++寫法。
char s[]
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#include <iostream>
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using namespace std;
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int main() {
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char s[] = "Hello World";
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for(int i = 0; i != 11; ++i) {
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cout << s[i];
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}
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cout << endl;
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for(int i = 0; i != 11; ++i) {
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cout << *(s + i);
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}
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cout << endl;
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}
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執行結果
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char *s
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#include <iostream>
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using namespace std;
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int main() {
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char *s = "Hello World";
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for(int i = 0; i != 11; ++i) {
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cout << s[i];
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}
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cout << endl;
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for(int i = 0; i != 11; ++i) {
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cout << *(s + i);
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}
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cout << endl;
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}
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執行結果
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但卻只有char *s可以使用*s++寫法。
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#include <iostream>
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using namespace std;
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int main() {
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char *s = "Hello World";
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while(*s)
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cout << *s++;
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}
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執行結果
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理由何在呢?
char s[]為陣列,雖然s = &s[0],但s是『常數』,恆等於&s[0]無法改變,但char *s為pointer,指向s[0],但卻是變數,可以任意改變,故可用*s++任意更改pointer值。
Conclusion
一般人很少分辨char s[]和char *s的差異,大部分狀況下用法相同,但char *s速度略快,因為不需copy的動作,且*s++為C語言常用的寫法,只有char *s才支援。
Reference
C Primer Plus 5/e中文版 p.480