关于const字符串数组存储一个奇怪的现象

[mcmay]

以下代码的作用是返回数组中最大值（如果是字符串数组则返回数组中最长的那个字符串，如果有几个等长字符串最长，则返回其中第一个的地址）

// ex6, ch 8
#include <iostream>
#include <cstring>

template <typename T>
T ret_max(T [], int);

template <> char * ret_max(char *[], int);

int main()
{
	using namespace std;
	int ar_int[6] = {3, 7, 2, 5, 6, 4};
	double ar_doub[4] = {2.1, 3.4, 1.8, 4.2};
	char * str[5] = {
		"I really can't understand you.",
		"You're my dearest baby.",
		"How much do you love me?",
		"I really can't understand you.",
		"I really can't understand you."
	};
	char * s;

	cout << "Max of the six integers: " << ret_max(ar_int, 6) << endl;;
	cout << "Max of the four integers: " << ret_max(ar_doub, 4) << endl;
	cout << "The five strings and their addresses:" << endl;
	for (int i = 0; i < 5; i++)
	{
		cout << str[i] << endl;
		cout << (unsigned)str[i] << endl;
	}
	cout << "The largest of the five and its address:" << endl;
	s = ret_max(str, 5);
	cout << s << endl;
	cout << (unsigned)s << endl;   // 用unsigned打印指针内容，请还有其他打印法可以显示0x进制的模式吗？

	return 0;
}

template <typename T>
T ret_max(T ar[], int n)
{
	T max = ar[0];
	for(int i = 0; i < n; i++)
		if(max < ar[i])
			max = ar[i];
	return max;
}

template <> char * ret_max(char * str[], int n)
{
	int i, max = 0;

	for(i = 0; i < n; i++)
		if(strlen(str[max]) < strlen(str[i]))
			max = i;

	return str[max];
}

运行结果：
Max of the six integers: 7
Max of the four integers: 4.2
The five strings and their addresses:
I really can't understand you.
4214892 // 这些长度一致的数组，竟然地址一样！
You're my dearest baby.
4214923
How much do you love me?
4214947
I really can't understand you.
4214892 // 同上
I really can't understand you.
4214892 // 同上
The largest of the five and its address:
I really can't understand you.
4214892

[brookmill]

引用 楼主 mcmay 的回复:

4214892 // 这些长度一致的数组，竟然地址一样！

第一，这些不是数组，而是指针。 第二，他们不只是长度一致，而是完全一模一样。如果改动其中一个而保持长度不变，你会看到地址就不一样了。

[brookmill]

char * str[5] = { "I really can't understand you.", "You're my dearest baby.", "How much do you love me?", "I really can't understand you.", "I really can't understand you." }; 这是一个数组，数组的每个元素都是char *。如果看一下sizeof(str)，应该是20，也就是5个指针的大小。 编译器实际上是在常量区保存了这些字符串，然后把他们的地址保存到str数组里面。 对于完全相同的字符串，大多数编译器会只保存一次。 简化一下： char *p1 = "abcd"; char *p2 = "abcd"; cout << (unsigned long)p1 << endl << (unsigned long)p2 << endl; // 看看是不是输出两个一样的值

[mcmay]

引用 2 楼 brookmill 的回复:

char * str[5] = { "I really can't understand you.", "You're my dearest baby.", "How much do you love me?", "I really can't understand you.", "I really can't understand you." }; 这是一个数组，数组的每个元素都是char *。如果看一下sizeof(str)，应该是20，也就是5个指针的大小。 编译器实际上是在常量区保存了这些字符串，然后把他们的地址保存到str数组里面。 对于完全相同的字符串，大多数编译器会只保存一次。 简化一下： char *p1 = "abcd"; char *p2 = "abcd"; cout << (unsigned long)p1 << endl << (unsigned long)p2 << endl; // 看看是不是输出两个一样的值

谢谢！原来如此！我实验过了，果然换一个字符就会导致地址变化。看来这也许是编译器的某种优化，让程序尽量少占空间。不过，如果程序真的需要将相同的字符串存放在不同地址，那就麻烦了。

[brookmill]

引用 3 楼 mcmay 的回复:

谢谢！原来如此！我实验过了，果然换一个字符就会导致地址变化。看来这也许是编译器的某种优化，让程序尽量少占空间。不过，如果程序真的需要将相同的字符串存放在不同地址，那就麻烦了。

常量区的字符串，只能读不能修改，好像没什么必要存放多个拷贝或者放到不同地址。 如果你想把字符串存放到其它地方，自己复制过去就行了。比如： char *p = "hello, world"; char str[128]; strcpy(str, p);

[FancyMouse]

编译器都有选项可以开关的。gcc是-fmerge-constants -fno-merge-constants，vc是/GF /GF-