使用memset填充数组显示垃圾

Question

I am stuck in a simple code snippet:

#include <iostream>
#include <stdio.h>
#include<cstring>
#include <algorithm>
int main()
{

        int l;
        std::cin >> l;
        char leading[l];
        //std::fill_n(leading, l, '0');
        memset(leading, '0', sizeof(char)*l);
        std::cout << "->" << leading << "<- strlen=" << strlen(leading) << std::endl;
}

Here is the result of multiple runs(same problem with std::fill_n ):

~/test$ g++ ptr.cpp 
~/test$ ./a.out 
3
->000*�<- strlen=6
~/test$ ./a.out 
7
->0000000<- strlen=7
~/test$ ./a.out 
6
->000000<- strlen=6
~/test$ ./a.out 
5
->00000<- strlen=6
~/test$ ./a.out 
2
->00@��<- strlen=6

Do I have to look for another job?

UPDATE: As I will be using this array later in a sprintf , I don't want to use a null terminator. Is there a better alternative?

Answer 1

'0' is not the same as '\\0' which is probably what you wanted. (No encoding system supported by C++ is allowed to set the character literal '0' to have the value 0.)

The behaviour on using strlen on this memory block will be undefined.

If you want to populate with the character 0, then remember to add a NUL-terminator to the memory block, in order for the C-type string functions (and << for that matter) to be usable.

Answer 2

Both strlen and operator<<(basic_ostream&, char*) have the precondition that the pointer arguments point to a null terminated string of characters. Your array is not null terminated and therefore you violate those preconditions. As a result, the behaviour of your program is undefined.

---

UPDATE: I don't with to use a null terminator. Is there a better alternative?

There is no need for an alternative for strlen since you already know the length: l .

For streaming a non-null-terminated character array, you can use an iterator:

std::ostream_iterator<char> it(std::cout);
std::copy(leading, std::end(leading), it);

---

PS.

 std::cin >> l; char leading[l]; 

This is ill-formed in C++. The size of an array must be a compile-time constant.

Answer 3

因为strlen和cout都依赖空终止符来指示字符串的结尾