How to convert string to char *?
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Thread: How to convert string to char *?

  1. #1
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    How to convert string to char *?

    Here is my code:
    Code:
    void func1(char * var1){
    ...
    }
    
    std::string str;
    
    func1( str... /* question here */ );
    How do I convert string to char *? I tried: str.c_str() , but c_str() returns a const. The function changes the string, so that doesn't work for me.

    Is there any better approach?
    Last edited by cpthk; August 18th, 2009 at 06:06 PM.

  2. #2
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    Re: How to convert string to char *?

    Why not change func1 such that it takes a std::string instead?

    If you cannot do that, one solution is to create an array of char of sufficient size, copy over the contents of the std::string (appending a null character), then pass a pointer to the first character of this new array to the function.
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  3. #3
    Join Date
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    Re: How to convert string to char *?

    Would this work for you?
    Code:
    void func1(std::string& var1){
    ...
    }
    
    std::string str;
    func1(str);

  4. #4
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    Re: How to convert string to char *?

    sorry, I am not allowed to do that. The function is written by another library. If I change the function, there are hundred of calls I need to update.

    @laserlight
    could you give me a code example?

  5. #5
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    Re: How to convert string to char *?

    Code:
    void func1(char * var1)
    is exactly like this or:
    Code:
    void func1(const char * var1)
    ?

    If second, then:
    Code:
    std::string str;
    func1(str.c_str());
    if first, then yes, you will need to make temporary char array, copy content into it and pass to function.

  6. #6
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    Re: How to convert string to char *?

    if first, then yes, you will need to make temporary char array, copy content into it and pass to function
    Or you can just do...

    Code:
    string s;
    
    ...
    
    vector<char> temp(s.length() + 1);
    temp.assign(s.begin(), s.end());
    temp.back() = '\0';
    
    func1(&temp[0]);
    ... letting vector cover the allocation for you.

  7. #7
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    Re: How to convert string to char *?

    You can even do it in one line!
    Code:
    vector<char> temp(s.c_str(), s.c_str() + s.length() + 1);
    "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
    Richard P. Feynman

  8. #8
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    Re: How to convert string to char *?

    I would be wary doing that. Notice the function takes a pointer to non-const. what if the function uses strcat and alters the length of the string that then passes the vectors capacity. Because you are accessing internals of the vector without going through vectors own functions for doing such, it could be problematical imo as the vector will lose internal consistency. This is one occaision where I may choose to manage the memory allocations personally or at the very least make sure I have reserved a fair bit of spare space in the vector. it all depends on exactly what the function will do, to which i could see no details.
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  9. #9
    Lindley is offline Elite Member Power Poster
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    Re: How to convert string to char *?

    If the function needs its argument to be of a certain size, then that's going to be a problem no matter *how* the memory is allocated. So you may as well use a vector sized appropriately.

  10. #10
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    Re: How to convert string to char *?

    Quote Originally Posted by JohnW@Wessex View Post
    You can even do it in one line!
    Code:
    vector<char> temp(s.c_str(), s.c_str() + s.length() + 1);
    Code:
    vector<char> temp(s.begin(), s.end());
    temp.push_back('\0');
    I personally prefer working with "end" iterators then using size type constructs. It is usually safer. forgetting the +1 after length can be easy. Also, its presence is (imo) confusing.

  11. #11
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    Re: How to convert string to char *?

    Quote Originally Posted by Russco View Post
    I would be wary doing that. Notice the function takes a pointer to non-const. what if the function uses strcat and alters the length of the string that then passes the vectors capacity.
    How is that any different than allocating too little space using manual memory management?
    Because you are accessing internals of the vector without going through vectors own functions for doing such,
    That is exactly the goal, and that is to use std::vector as just a buffer. It is guaranteed that a vector stores its buffer in contiguous memory, just like any array.
    This is one occaision where I may choose to manage the memory allocations personally or at the very least make sure I have reserved a fair bit of spare space in the vector. it all depends on exactly what the function will do, to which i could see no details.
    You are not improving anything by managing your own memory in this case. Whatever memory you reserved doing manual memory management will encounter the same issues if you don't size your vector appropriately.

    Whatever you did "manually", you can do the same thing with vector. Give us a case where you have managed memory manually (using new[] and delete[]) where a vector couldn't be used.

    Regards,

    Paul McKenzie

  12. #12
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    Re: How to convert string to char *?

    Code:
    #include <string>
    #include <iostream>
    
    void func1(char* var1){}
    
    char* cppstr_to_cstr(const std::string& rstr)
    {
        char* pcstr = (char*) malloc ( rstr.size() + 1 );
        char* pcstr_r = pcstr;
        
        for (int i = 0; i < rstr.size(); i++)
        {
            *pcstr = rstr[i];
            pcstr++;
        }
        *pcstr = '\0';
        
        return pcstr_r;
    }
    
    int main()
    {
        std::string str = "Hello World!";
        
        char* pcstr = cppstr_to_cstr( str );
        
        func1( pcstr );
        
        printf("&#37;s", pcstr);
        
        free ( pcstr );
        pcstr = 0;
        
        system("pause");
    }
    Last edited by mwoods; August 20th, 2009 at 09:12 PM.

  13. #13
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    Re: How to convert string to char *?

    Code:
        char* pcstr = (char*) malloc ( rstr.size() + 1 );
    Who is responsible for cleaning up the memory? Why introduce dynamic allocation? The solutions that were given using vector are far superior and safer.
    Code:
    string s;
    //...
    vector<char> temp(s.c_str(), s.c_str() + s.length() + 1);
    char *myptr = &temp[0];
    All of the code you wrote is within those last two lines.

    Regards,

    Paul McKenzie
    Last edited by Paul McKenzie; August 20th, 2009 at 08:49 PM.

  14. #14
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    Re: How to convert string to char *?

    Quote Originally Posted by Paul McKenzie View Post
    Who is responsible for cleaning up the memory?
    Oh my god, I so can't believe you just said that!
    Attached Images Attached Images  

  15. #15
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    Re: How to convert string to char *?

    Quote Originally Posted by mwoods
    Oh my god, I so can't believe you just said that!
    I can't believe that you can't believe that Paul McKenzie wrote that. If ownership is not clear, resource leaks are bound to happen.
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