Possible memory leak.

This is a discussion on Possible memory leak. within the C++ Programming forums, part of the General Programming Boards category; I'm doing an exercise from C++ Primer book, and I suspect that this bit of code I've written is leaking ...

  1. #1
    msh
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    Possible memory leak.

    I'm doing an exercise from C++ Primer book, and I suspect that this bit of code I've written is leaking memory.

    Code:
    struct Stringy {
        char* str;
        int ct; // lenght w/o '\0';
    };
    
    void set(Stringy& st, const char s[]);
    
    // main()
    
    void set(Stringy& st, const char s[]) {
        using namespace std;
        
        int len = strlen(s);
        char* t = new char[len + 1];
        
        st.ct = len;    
        st.str = t;
        strcpy(st.str, s);
    }
    Now, the way I understand this is that every time set() is called new block of memory is allocated for t and then st.str is set to point to this block. My concern is that the block st.str previously pointed to is, to the best of my understanding, never released.

    So my questions are (1) do I understand the situation correctly and if yes (2) what can be done to remedy it?

  2. #2
    C++11 User Tux0r's Avatar
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    Quote Originally Posted by msh View Post
    I'm doing an exercise from C++ Primer book, and I suspect that this bit of code I've written is leaking memory.

    Code:
    struct Stringy {
        char* str;
        int ct; // lenght w/o '\0';
    };
    
    void set(Stringy& st, const char s[]);
    
    // main()
    
    void set(Stringy& st, const char s[]) {
        using namespace std;
        
        int len = strlen(s);
        char* t = new char[len + 1];
        
        st.ct = len;    
        st.str = t;
        strcpy(st.str, s);
    }
    Now, the way I understand this is that every time set() is called new block of memory is allocated for t and then st.str is set to point to this block. My concern is that the block st.str previously pointed to is, to the best of my understanding, never released.

    So my questions are (1) do I understand the situation correctly and if yes (2) what can be done to remedy it?
    in your set function, use delete[] st.str;

    Code:
    #include <iostream>
    #include <cstring>
    
    struct Stringy {
        char* str;
        int ct; // lenght w/o '\0'
    };
    
    void set(Stringy& st, const char s[]) {
    	st.ct=strlen(s);
    	delete[] st.str;
    
    	st.str=new char[st.ct+1];
    	strcpy(st.str,s);
    }
    
    int main() {
    	Stringy test={NULL,0};
    	set(test,"squirrel");
    
    	std::cout<<test.str<<std::endl;
    }
    Last edited by Tux0r; 07-21-2009 at 06:39 AM.

  3. #3
    msh
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    Isn't the following undefined behavior, at least on the first run? Since I'm using delete on memory I did not allocate with new.
    Code:
    delete[] st.str;

  4. #4
    C++11 User Tux0r's Avatar
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    Quote Originally Posted by msh View Post
    Isn't the following undefined behavior, at least on the first run? Since I'm using delete on memory I did not allocate with new.
    Code:
    delete[] st.str;
    In main() I initialized str to NULL

  5. #5
    and the hat of sweating
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    Why not make Stringy a class with constructors & destructors that can properly manage the member variables (and make the members private)?
    "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008

    "the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010

  6. #6
    C++11 User Tux0r's Avatar
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    Quote Originally Posted by msh View Post
    Isn't the following undefined behavior, at least on the first run? Since I'm using delete on memory I did not allocate with new.
    Code:
    delete[] st.str;
    Also this is how you do it in C++, if you use classes

    Code:
    #include <iostream>
    #include <cstring>
    
    class Stringy {
        public:
            char* str;
            int len;
    
            Stringy(): str(),len() {}
    
            void set(const char*);
    };
    
    void Stringy::set(const char *s) {
    	len=strlen(s);
    	delete[] str;
    
    	str=new char[len+1];
    	strcpy(str,s);
    }
    
    int main() {
    	Stringy test; //default constructor called here
    
    	test.set("squirrel");
    	std::cout<<test.str<<std::endl;
    }
    edit: although note this is a poorly written class, it merely gives you an idea of basic init with classes
    Last edited by Tux0r; 07-21-2009 at 06:54 AM.

  7. #7
    msh
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    Quote Originally Posted by Tux0r View Post
    In main() I initialized str to NULL
    Ah. Forgot that it's OK to use delete on NULL pointers.

    (Classes are not available at this point. The book introduces them only at about half way through.)

  8. #8
    C++11 User Tux0r's Avatar
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    Quote Originally Posted by msh View Post
    (Classes are not available at this point. The book introduces them only at about half way through.)
    Ok then you just got a little head start

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Tux0r View Post
    Also this is how you do it in C++, if you use classes

    Code:
    #include <iostream>
    #include <cstring>
    
    class Stringy {
        public:
            char* str;
            int len;
    
            Stringy(): str(),len() {}
    
            void set(const char*);
    };
    
    void Stringy::set(const char *s) {
    	len=strlen(s);
    	delete[] str;
    
    	str=new char[len+1];
    	strcpy(str,s);
    }
    
    int main() {
    	Stringy test; //default constructor called here
    
    	test.set("squirrel");
    	std::cout<<test.str<<std::endl;
    }
    edit: although note this is a poorly written class, it merely gives you an idea of basic init with classes
    Forgot to delete in destructor.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #10
    C++11 User Tux0r's Avatar
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    heh

  11. #11
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    >> this is how you do it in C++
    In C++ I'd use an existing string class, and I'd use vector instead of new[]/delete[] anyway. But of course those are also beyond the scope of msh's current learning.

    >> Forgot to delete in destructor.
    And forgot the rule of three as well.

    BTW, is this C++ Primer by Lippman, et al or C++ Primer Plus by Prata?

  12. #12
    C++11 User Tux0r's Avatar
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    lol I was not trying to write a Tux0r::String I was only showing msh how to not have to init structs manually...

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