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);
}

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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?

Quote:

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Originally Posted by msh

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;
}

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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:

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delete[] st.str;

---

Quote:

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Originally Posted by msh

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

Why not make Stringy a class with constructors & destructors that can properly manage the member variables (and make the members private)?

Quote:

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Originally Posted by msh

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;
}

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edit: although note this is a poorly written class, it merely gives you an idea of basic init with classes

Quote:

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Originally Posted by Tux0r

In main() I initialized str to NULL

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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.)

Quote:

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Originally Posted by msh

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

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Ok then you just got a little head start ;)

Quote:

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Originally Posted by Tux0r

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

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Forgot to delete in destructor.

:) heh

>> 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?

lol I was not trying to write a Tux0r::String I was only showing msh how to not have to init structs manually...