How to use strtok twice in the same program?

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# 1  
How to use strtok twice in the same program?

       string str1(" 1 2 3 4 512543 ");
                 string str2;
                cout << "str2:" <<str2 <<endl;
                p1=strtok((char *)str1.c_str()," ");
                cout << "val of p1 " << p1 << endl;
                 p1=strtok(NULL," ");
                cout << "size of v1 " << v1.size() <<endl;
                p2=strtok((char *)str2.c_str()," ");
                cout << "str2:" <<str2 <<endl;
                        cout << "val of p2 " << p2 <<endl;
                        p2=strtok(NULL," ");

                cout << "size of v2" << v2.size() <<endl;

I get the following o/p.

str2: 1 2 3 4 512543 
val of p1 1
val of p1 2
val of p1 3
val of p1 4
val of p1 512543
size of v1 5
str2: 1234512543
val of p2 1
after tok p2  
size of v21

I want str2 also to be tokenized. But strtok after reaching NULL once it returns only NULL pointer everytime when it is accessed using NULL.

How can I tokenize str2 now?

# 2  
I don't understand the necessity of using strtok() for your problem. Anyway, the answer is just all over the manual for strtok(3):
Avoid using these functions. If you do use them, note that:

These functions modify their first argument.

These functions cannot be used on constant strings.

The identity of the delimiting character is lost.

The strtok() function uses a static buffer while parsing, so it's not thread safe. Use strtok_r() if this matters to you.
To make your code work, you could trick the compiler this way:
// ...
       string str1(" 1 2 3 4 512543 ");
                 string str2;
                        str2.insert(0, ""); // now, you tell me why this apparently solves your problem!
                cout << "str2:" <<str2 <<endl;
                p1=strtok((char *)str1.c_str()," ");
// ...

# 3  
How to use strtok twice in the same program?

Yes. It worked!!!
But how did the insertion of
str2.insert(0, "");

solved the problem?
The manpages also suggests that it is not advisable to use.
# 4  
better use strtok_r
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