char* issue in C++ [closed] - c++

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Why this fragment of code does not work? I know that all entered strings have length less than 20 symbols.I do not use std::string because I want to learn how to use char*
#include <map>
#include <stdio.h>
using namespace std;
map <char*,int> cnt;
int main()
{
char ans[20];
int n,mx = 0;
scanf("%d\n",&n);
for ( int i = 1; i <= n; i++){
char str[20];
gets(str);
cnt[str]++;
}
for ( auto i = cnt.begin(); i != cnt.end(); i++ )
puts(i->first);
}

Answer 1

Let's be clear that your code has a lot of undefined behavior. I tried running your code and here is what I saw on my machine. You should tell us what your behavior was though because it's impossible to say what's going on for you otherwise.
First off, here was my program input.
3
hello
world
cat
And the output...
cat
char str[20] is a memory address, and that address is being reused by the compiler. Let's say that memory address is 0xABCD.
So on the first iteration, the map contains one element which is { 0xABCD, 1 }. On the second iteration it contains the same element with its value incremented, {0xABCD, 2}. On the third iteration it contains {0xABCD, 3}. Then when you go to print the map, it finds only one element in the map, and prints that memory address. This memory address happens to contain the word "cat", so it prints cat.
But this behavior is not reliable. The array char str[20] doesn't exist outside of the for loop, so sticking it into map <char *, int> cnt and even worse printing the array outside the loop are both undefined behavior.
If you want your code to work, I suppose you could do this....
for ( int i = 1; i <= n; i++){
char * str = new char[20];
gets(str);
cnt[str]++;
}
for ( auto i = cnt.begin(); i != cnt.end(); i++ )
puts(i->first);
for ( auto i = cnt.begin(); i != cnt.end(); i++ )
delete[](i->first);
But really, the correct strategy here is to either....
1) Use std::string
or
2) Don't use std::map
If you want to use C strings beyond converting them to std::string, then program without the use of the C++ std library. Stick to the C standard library.

Answer 2

Seems like cnt is std::map<char*, ...>. When you do cnt[str] you use pointer to local variable str as key, but str is only valid during single iteration. After that str is freed (semantically, optimizer may reuse it, but it is irrelevant here) and pointer to it is no longer valid.

Answer 3

It's very simple: when you allocate a C-style array as a local variable (char str[20];), it is allocated on the stack. It behaves just like any other object that you allocate as a local variable. And when it falls out of scope, it will be destroyed.
When you try to pass the array to the map in cnt[str], the array name decays to a pointer to the first element (it implicitely converts an expression of type char[20] into an expression of type char*). This is something radically different than an array. The map only ever sees this single pointer and stores it as the key. The map does not dereference the pointer to find out what's behind it, it just uses the memory location.
To fix your code, you need to do two things:
You need to allocate memory for your strings on the heap, so that the char* remains valid after the end of the scope. The easiest way to do this is to use the getline() or getdelim() functions available in the POSIX-2008 standard: These beautiful two functions will actually do the malloc() call for you. However, you still need to remember to free the string afterwards.
Making the map aware that you are talking about strings and not about memory addresses is much harder to achieve. If you must use a map, you likely need to define your own std::string-like wrapper class. But I guess, since you are playing around with the char* to learn their use, it would be more prudent to use some other kind of list and program the logic to check whether the given string is already in the list. Could be an array of char*, probably sorted to save lookup time, or a linked list, or whatever you like. For ease, you can just use an std::vector<char*>, but don't forget to free your strings before letting the vector fall out of scope.