[c++] "warning: statement has no effect"

[c++] "warning: statement has no effect"

Mark Oates

Member #1,146

March 2001

I'm getting this message using a for loop that actually does have an affect. How can I turn off this warning?

Oh, and while I'm at it how do I turn off "multiple definition" errors?

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Matthew Leverton

Supreme Loser

January 1999

Write for (a;b;c); as for (a;b;c) {}.

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Mark Oates

Member #1,146

March 2001

It already is. :-/ The for loop is from tinyXML's recommended method for reading from the document:

TiXmlElement* gimp_element;

for(gimp_element; gimp_element; gimp_element=gimp_element->NextSiblingElement())
{
  ...
}

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ReyBrujo

Moderator

January 2001

Quote:

Oh, and while I'm at it how do I turn off "multiple definition" errors?

Correcting the code. That usually means you have defined a variable in a header that is being included in several. Declare the variable as extern in the header, and define it once in a source file without the modifier.

And try initializing the iterator variable.

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RB
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Matthew Leverton

Supreme Loser

January 1999

Change it to:

for(gimp_element; gimp_element; (gimp_element=gimp_element->NextSiblingElement()))
{
}

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Mark Oates

Member #1,146

March 2001

ah-hah!

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Matthew Leverton

Supreme Loser

January 1999

And by the way, if your for loop has no affect, it might suffer from schizophrenia. :-/

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ReyBrujo

Moderator

January 2001

{"name":"effect_an_effect.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/c\/cc83c72c4c0078e428278e3a82ebf2c4.png","w":340,"h":423,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/c\/cc83c72c4c0078e428278e3a82ebf2c4"}
XKCD

--
RB
光子「あたしただ…奪う側に回ろうと思っただけよ」
Mitsuko's last words, Battle Royale

Mark Oates

Member #1,146

March 2001

Quote:

it might suffer from schizophrenia.

it does.

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Kitty Cat

Member #2,815

October 2002

Quote:

for(gimp_element; gimp_element; gimp_element=gimp_element->NextSiblingElement())

The first part of the for statement (where it sets the initial pre-loop setting) has no effect. That for loop would be rewritten as:

TiXmlElement* gimp_element;

gimp_element;
while(gimp_element) {
    ...
    gimp_element=gimp_element->NextSiblingElement();
}

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Johan Halmén

Member #1,550

September 2001

http://www.allegro.cc/files/attachment/593802

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Matthew Leverton

Supreme Loser

January 1999

Quote:

The first part of the for statement

Unless there's a clever macro going on; after all, this is C++.

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Tobias Dammers

Member #2,604

August 2002

I don't think you even need a macro to really fsck things up badly.

1struct GimpElement {
2  mutable int counter;
3 
4  GimpElement(int aCounter): counter(aCounter) {}
5  GimpElement(): counter(0) {}
6  operator bool() const { return (++counter < 42); }
7  GimpElement NextSiblingElement() const { return GimpElement(counter + 1); }
8  GimpElement& operator=(const GimpElement& rhs) { counter = rhs.counter + 1; }
9};
10 
11int main() {
12  GimpElement gimp_element;
13  for (gimp_element; gimp_element; gimp_element = gimp_element->NextSiblingElement())
14    ;
15  return 0;
16}

Untested though.

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CGamesPlay

Member #2,559

July 2002

Looks like you increment twice

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