unreferenced local variable

Discussion in 'C++' started by Alzane, Nov 29, 2004.

  1. Alzane

    Alzane Guest

    I'm new to C++ programming. I have an exercise that I have written
    code for but getting warnings. Can I get some help?


    #include "stdafx.h"
    #include <string.h>
    #include <stdio.h>
    #include <stdlib.h>


    int main(int argc, char* argv[])

    {
    char *StrNam1;
    char *StrNam2;

    StrNam2 = (char *) malloc (strlen (StrNam2) + 1);
    sprintf("%s%s", strcat
    ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D",
    "A3D6F20D-7091"));


    return 0;
    }

    Compiling...
    Concatenate.cpp
    C:\Program Files\Microsoft Visual
    Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
    'StrNam1' : unreferenced local variable
    C:\Program Files\Microsoft Visual
    Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
    variable 'StrNam2' used without having been initialized
    Linking...

    Concatenate.exe - 0 error(s), 2 warning(s)
    Alzane, Nov 29, 2004
    #1
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  2. Alzane

    Ron Natalie Guest

    Alzane wrote:
    > I'm new to C++ programming. I have an exercise that I have written
    > code for but getting warnings. Can I get some help?
    >
    >
    > #include "stdafx.h"


    BOGUS include file.


    >
    > int main(int argc, char* argv[])
    >
    > {
    > char *StrNam1;

    You indeed don't use this anywhere, causing the first warning.

    > char *StrNam2;
    >
    > StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


    At this point StrNam2 is set to some indeterminate value. What do
    you think passing it to strlen is going to do? The length of WHAT
    string?

    > sprintf("%s%s", strcat
    > ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D",
    > "A3D6F20D-7091"));
    >

    This isn't how sprintf works at all. The first arg to sprintf has
    to be a char* that points to some allcoated memory that the printf()
    results will be written too.

    The second arg is the format string. And you strcat doesn't work
    the way you think it should either.

    At this point, you probably should stay away from C's hiddeous attempt
    to implement strings as character arrays and just use C++'s string type.

    #include <string>
    int main() {
    std::string StrNam2 = "..%2F..dkafjkjdflasdkjf";
    StrNam2 += "A3D6F20D-7091";

    }
    Ron Natalie, Nov 29, 2004
    #2
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  3. Alzane wrote:
    > I'm new to C++ programming. I have an exercise that I have written
    > code for but getting warnings. Can I get some help?
    >
    >
    > #include "stdafx.h"
    > #include <string.h>
    > #include <stdio.h>
    > #include <stdlib.h>
    >
    >
    > int main(int argc, char* argv[])
    >
    > {
    > char *StrNam1;


    You declared the pointer 'StrNam1' but never use it in the program.
    That's why the compiler warns you.

    > char *StrNam2;
    >
    > StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


    What are you trying to do here? 'StrNam2' has no value given to it.
    You're trying to find its length by calling 'strlen'. That's the
    second warning you get. Anyway, that's not the right way to manage
    your memory. If you want to allocate some memory and store the pointer
    to it in 'StrNam2', you need to give it some meaningful size. What is
    the purpose of 'StrNam2'?

    > sprintf("%s%s", strcat
    > ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D",
    > "A3D6F20D-7091"));


    'sprintf' is supposed to have the string where you want the result as
    the very first argument. The format is the _second_ argument. RTFM.

    'strcat' is used incorrectly. If you just want to have two literals
    live on two different lines of code but represent the same string, you
    can simply leave whitespace between them:

    "..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D"
    "A3D6F20D-7091"

    No commas, no 'strcat' calls. 'strcat' requires the first argument to
    be the _resulting_ buffer. If you pass a literal as the first argument,
    you're asking for trouble. RTFM.

    >
    >
    > return 0;
    > }
    >
    > Compiling...
    > Concatenate.cpp
    > C:\Program Files\Microsoft Visual
    > Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
    > 'StrNam1' : unreferenced local variable
    > C:\Program Files\Microsoft Visual
    > Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
    > variable 'StrNam2' used without having been initialized
    > Linking...
    >
    > Concatenate.exe - 0 error(s), 2 warning(s)


    V
    Victor Bazarov, Nov 29, 2004
    #3
  4. Alzane

    Mike Wahler Guest

    "Alzane" <> wrote in message
    news:...
    > I'm new to C++ programming. I have an exercise that I have written
    > code for but getting warnings. Can I get some help?
    >
    >
    > #include "stdafx.h"


    This is a nonstandard header. Please omit such from
    code posted here. You don't need this anyway.

    > #include <string.h>
    > #include <stdio.h>
    > #include <stdlib.h>
    >
    >
    > int main(int argc, char* argv[])
    >
    > {
    > char *StrNam1;
    > char *StrNam2;


    You've just defined two pointers. Since you did not initialize them,
    their values are indetermindate, unknown. I.e. these pointers don't
    point anywhere.

    >
    > StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


    Here you pass an ininitalizaed pointer ('StrNam2') to a function
    ('strlen()'), which expects a valid pointer to a zero-terminated
    array of characters. The resulting behavior is undefined. If you
    want to compute the length of a string, first you need a string.

    You also need to check 'malloc()'s return value to see if it
    failed (returns NULL in that case).

    You never subsequently use 'StrNam2' in your program anyway,
    so I'm not sure what your trying to do. Perhaps if you
    explain that, I could offer more specific advice.

    > sprintf("%s%s", strcat
    > ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D",
    > "A3D6F20D-7091"));


    Here's another case of undefined behavior. 'strcat()' will attempt
    to modify the string pointed to by its first parameter. But you give
    a pointer to a string literal, which the language prohibits modifying.

    Another 'undefined' aspect of this is that you've given 'printf()'
    two format specifiers ('%s'), but only one corresponding argument.

    If you want to concatentate two strings, you need a place to store
    the resulting string. (This could be the string being lengthened,
    if sufficient storage has been allocated for it).

    >
    >
    > return 0;
    > }
    >
    > Compiling...
    > Concatenate.cpp
    > C:\Program Files\Microsoft Visual
    > Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
    > 'StrNam1' : unreferenced local variable


    This is harmless. It simply means you've created an object an
    never used it. Use modern C++ and forget that low-level memory
    management you've tried above.


    -Mike

    > C:\Program Files\Microsoft Visual
    > Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
    > variable 'StrNam2' used without having been initialized


    This is the real problem.


    Since none of what you wrote above enough sense for me to
    try to guess what you're trying to do, I can only offer this
    simple C++ program which outputs the result of concatenating
    two strings.


    #include <iostream>
    #include <string>

    int main()
    {
    std::string StrNam1("Hello ");
    std::string StrNam2("world");
    std::cout << StrNam1 + StrNam2 << '\n';
    return 0;
    }

    -Mike
    Mike Wahler, Nov 29, 2004
    #4
  5. Alzane

    Default User Guest

    Alzane wrote:

    > I'm new to C++ programming. I have an exercise that I have written
    > code for but getting warnings. Can I get some help?
    >
    >
    > #include "stdafx.h"


    The above header is non-standard.

    > #include <string.h>
    > #include <stdio.h>
    > #include <stdlib.h>


    While technically C++ headers, they are provided mostly for
    compatibility with C. You should probably not be using them.

    >
    > int main(int argc, char* argv[])


    As you don't use the args in main(), you should leave them out.

    > {
    > char *StrNam1;
    > char *StrNam2;


    It's a poor idea for a beginner to use pointers in learning programs.

    > StrNam2 = (char *) malloc (strlen (StrNam2) + 1);


    StrNam2 was not initialized. What did you think the result of getting
    the string length of it would be? What it is in Undefined Behavior, a
    Very Bad Thing.

    Also, malloc() is generally not used in C++ programming.

    > sprintf("%s%s", strcat
    > ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D",
    > "A3D6F20D-7091"));


    You don't know how to use sprintf() at all. The arg count is wrong.
    Also, your use of strcat() modifies a string literal. Undefined
    Behavior again.

    >
    > return 0;


    You got this right.

    > }
    >
    > Compiling...
    > Concatenate.cpp
    > C:\Program Files\Microsoft Visual
    > Studio\MyProjects\dash2hex\Concatenate.cpp(10) : warning C4101:
    > 'StrNam1' : unreferenced local variable


    You didn't use StrNam1.

    > C:\Program Files\Microsoft Visual
    > Studio\MyProjects\dash2hex\Concatenate.cpp(13) : warning C4700: local
    > variable 'StrNam2' used without having been initialized


    This is the far more dangerous problem.


    What book are you using? This is a mishmashed, dangerous piece of
    essentially C code.

    Get a good book, one using modern C++, then read it. Otherwise, you're
    wasting your time and ours.



    Brian
    Default User, Nov 29, 2004
    #5
  6. Alzane

    Method Man Guest

    "Alzane" <> wrote in message
    news:...
    > I'm new to C++ programming. I have an exercise that I have written
    > code for but getting warnings. Can I get some help?
    >
    >
    > #include "stdafx.h"
    > #include <string.h>
    > #include <stdio.h>
    > #include <stdlib.h>
    >
    >
    > int main(int argc, char* argv[])
    >
    > {
    > char *StrNam1;
    > char *StrNam2;
    >
    > StrNam2 = (char *) malloc (strlen (StrNam2) + 1);
    > sprintf("%s%s", strcat
    > ("..%2F..%2FCommon%2FASP%2Fquot_choose_lob.asp%3FSID%3D",
    > "A3D6F20D-7091"));
    >
    >
    > return 0;
    > }



    A couple more suggestions:

    1. Don't cast the result of malloc, it can be unsafe.
    2. For every malloc(), make sure to have a corresponding free() call. It's
    better to be safe and manage your own memory.

    Note: The above looks like C code. A good book on C or C++ would help you
    with your exercise much more.
    Method Man, Nov 30, 2004
    #6
  7. Alzane

    Ron Natalie Guest

    Method Man wrote:
    =
    >
    > A couple more suggestions:
    >
    > 1. Don't cast the result of malloc, it can be unsafe.


    You have to cast the result of malloc. There is no implicit
    conversion from void* in C++.


    > 2. For every malloc(), make sure to have a corresponding free() call. It's
    > better to be safe and manage your own memory.


    True, but it would be better to avoid user-managed dynamic allocations
    here entirely. That would have avoided 90% of the problems.
    Ron Natalie, Nov 30, 2004
    #7
  8. Alzane

    Alzane Guest

    Thanks to you all for you explanations. I'm going through this book
    called "C Programming for the Absolute Beginner", which as you can see
    is definitely me, but I have this person who trying to get me up to
    speed with some exercises. I agree I should study more. If you have
    any suggestion of a good C Programming book, please advise.
    Alzane, Nov 30, 2004
    #8
  9. Alzane

    Default User Guest

    Ron Natalie wrote:

    > Method Man wrote:
    > =
    > >
    > > A couple more suggestions:
    > >
    > > 1. Don't cast the result of malloc, it can be unsafe.

    >
    > You have to cast the result of malloc. There is no implicit
    > conversion from void* in C++.


    Yep, MM is thinking of C. In C++, not only is the cast required but the
    "C problem" of implicit declaration of undeclared functions doesn't
    exist either.

    > > 2. For every malloc(), make sure to have a corresponding free()
    > > call. It's better to be safe and manage your own memory.

    >
    > True, but it would be better to avoid user-managed dynamic allocations
    > here entirely. That would have avoided 90% of the problems.


    I agree. Newbies to C++ should be starting out with containers. Once a
    firm grasp of programming is established, then dynamic memory can be
    tackled.



    Brian
    Default User, Nov 30, 2004
    #9
  10. Alzane

    Default User Guest

    Alzane wrote:

    > Thanks to you all for you explanations. I'm going through this book
    > called "C Programming for the Absolute Beginner", which as you can see
    > is definitely me, but I have this person who trying to get me up to
    > speed with some exercises. I agree I should study more. If you have
    > any suggestion of a good C Programming book, please advise.


    Why are you posting to a C++ newsgroup, when you are attempting to
    learn C? Many of the comments you received were inappropriate in the
    context of a review of a C program (although many others were the same).

    You should be using comp.lang.c.




    Brian
    Default User, Nov 30, 2004
    #10
  11. Alzane

    Method Man Guest

    "Default User" <> wrote in message
    news:...
    > Ron Natalie wrote:
    >
    > > Method Man wrote:
    > > =
    > > >
    > > > A couple more suggestions:
    > > >
    > > > 1. Don't cast the result of malloc, it can be unsafe.

    > >
    > > You have to cast the result of malloc. There is no implicit
    > > conversion from void* in C++.

    >
    > Yep, MM is thinking of C. In C++, not only is the cast required but the
    > "C problem" of implicit declaration of undeclared functions doesn't
    > exist either.
    >


    Thanks, I wasn't aware of that. Personally, I've never had to use malloc()
    in a C++ program yet.
    Method Man, Nov 30, 2004
    #11
  12. Alzane

    Method Man Guest

    "Default User" <> wrote in message
    news:...
    > Alzane wrote:
    >
    > > Thanks to you all for you explanations. I'm going through this book
    > > called "C Programming for the Absolute Beginner", which as you can see
    > > is definitely me, but I have this person who trying to get me up to
    > > speed with some exercises. I agree I should study more. If you have
    > > any suggestion of a good C Programming book, please advise.

    >
    > Why are you posting to a C++ newsgroup, when you are attempting to
    > learn C? Many of the comments you received were inappropriate in the
    > context of a review of a C program (although many others were the same).
    >
    > You should be using comp.lang.c.
    >


    Yes. C and C++ are two different languages and it is a waste of people's
    time to analyze code for one language when you are referring to another.
    Method Man, Nov 30, 2004
    #12
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