strcpy、strcat、memcpy、memset、strlen简易实现和注意问题

strcpy

代码

char* mystrcpy(char* dst, const char* src) {
	if((dst == nullptr)||(src == nullptr)) return nullptr; 
	char* ret = dst;
	while ((*dst++ = *src++) != '\0');
	return ret;
}
char* mystrcpy(char* dst, const char* src) {
	if (dst == nullptr || src == nullptr) return nullptr;
	return (char*)mymemcpy(dst, src, mystrlen(src)+1);
}

注意问题

1. const修饰源字符串
2. 空指针检查
3. ret保存原始的dst指针，即字符串开头
4. 返回char*，为了支持链式表达式如int l=strlen(strcpy(strA,strB));
5. dst和src所指的内存区域不可以重叠
6. dst需要分配内存

memcpy

代码

#include <iostream>
using namespace std; 
void* mymemcpy(void* dest, const void* src, size_t n) {
	if (dest == nullptr || src == nullptr || n == 0) return nullptr;
	void* ret = dest;
	if (dest < src || (char*)src + n < dest) {
		size_t count = n / sizeof(size_t);
		while (count--) {
			*(size_t*)dest = *(size_t*)src;
			dest = (size_t*)dest + 1;
			src = (size_t*)src + 1;
		}
		count = n % sizeof(size_t);
		while (count--) {
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	else {
		dest = (char*)dest + n - 1;
		src = (char*)src + n - 1;
		while (n--) {
			*(char*)dest = *(char*)src;
			dest = (char*)dest - 1;
			src = (char*)src - 1;
		}
	}
	return ret;
}
int main() {
	char dest[] = "12345678901234567890";
	char src[] = "123456789123456789";
	my_memcpy(dest, src, strlen(src) + 1);
	cout << dest << endl;

	return 0;
}

strcat

代码

char* mystrcat(char* dst, const char* src) {
	if (dst == nullptr || src == nullptr) return nullptr;
	char* pdst = dst;
	while (*pdst != '\0') ++pdst;
	while (*pdst++ = *src++);
	return dst;
}

注意问题

1. dst要有容纳src的长度
2. dst和src所指的内存区域不可以重叠

strlen

代码

size_t mystrlen(const char* ptr) {
	if (ptr == nullptr) return 0;
	size_t ret = 0;
	while (*ptr++ != '\0') ++ret;
	return ret;
}

memset

代码

void* mymemset(void* ptr, int ch, size_t n) {
	if (ptr == nullptr || n <= 0) return nullptr;
	char* pptr = (char*)ptr;
	while (n--) *pptr++ = ch;
	return ptr;
}