How To Debug Compiler Errors

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Cygwin!

Introduction

Explaining compiler error messages is difficult because there are so many ways to make errors. Also, many different mistakes can cause the same error message. Thus, this "How To" does not attempt to explain all possible compile-time errors and all possible causes. Instead, it covers only the most common problems and the error messages they produce.

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Anatomy of an Error Message

Debugging compiler messages means that you have to read them and extract the important information from them. For example, the following error message:

fiddle.cpp: In function `int main()':
fiddle.cpp:6: error: parse error before `return'

tells you several important things:

  1. the name of the program
    2. fiddle.cpp: In function `int main()':
  2. the function where the error occurs
    3. fiddle.cpp: In function `int main()':
  3. the line number of the error
    4. fiddle.cpp:6: error: parse error before `return'
  4. the error message
    5. fiddle.cpp:6: error: parse error before `return'

Thus, even scary-looking error messages can give you important clues for fixing problems.

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First Steps

The first step to take when analyzing compiler messages is to look for the line number of the first error message. This tells you the line where the compiler first got confused. Look at this line in your code and try to spot the problem. If you cannot see a problem on that line, look for a problems on lines just before the reported line.

Most times, you will spot the error. If you do not, then compare the first error message with those listed on this page. The comments may help you find the problem. If not, you should bring your code to the instructor and ask for assistance.

Note that you should look at the first error and ignore the rest. Often the first error causes all the rest of the errors. Reading any error beyond the first is usually a waste of time.

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Some Common Error Messages

The following sections describe common error messages and their causes. These messages include:

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missing terminating " character

This error is usually caused by a missing double quote ("). For example, the following code:

#include <iostream>
using namespace std;

int main() {
    string message = "Hello world!;
    cout << message << endl;
    return 0;
}

generates the following error:

fiddle.cpp:5:22: missing terminating " character
fiddle.cpp: In function `int main()':
fiddle.cpp:6: error: parse error before `<<' token

The fix, of course, is to add the missing double quote.

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parse error at ..., parse error before ...

This is a general-purpose syntax error and is often caused by:

  • A missing semicolon (;)
  • A missing curly bracket
  • A missing << between terms of cout
  • A missing >> between terms of cin

This error is frequently caused by a missing semicolon. For example, the following code:

#include <iostream>
using namespace std;

int main() {
    cout << "Hello world!" << endl
    return 0;
}

generates the following error:

fiddle.cpp: In function `int main()':
fiddle.cpp:6: error: parse error before `return'

Adding a semicolon (;) at the end of the line cout << "Hello world!" << endl will get rid of this error.

Notice that the error refers to line 6, but the error is actually on the previous line. This is quite common. Since C++ compilers are lenient about where you place whitespace, the compiler treats line 5 and line 6 as a single line that reads as follows:

cout << "Hello world!" << endl return 0;

Since this is incorrect syntax, the compiler complains.

Another cause of these types of errors is a missing curly bracket. For example, the following code:

#include <iostream>
using namespace std;

int main() {
    if (true) {
        cout << "Hello world!" << endl;
    return 0;
}

generates the following error:

fiddle.cpp: In function `int main()':
fiddle.cpp:8: error: parse error at end of input

Because there is no closing curly bracket for the if statement, the compiler thinks the curly bracket that terminates the main() function actually terminates the if statement. When it does not find a curly bracket on line 8 of the program to terminate the main function, it complains. Adding the closing curly bracket fixes the problem

#include <iostream>
using namespace std;

int main() {
    if (true) {
        cout << "Hello world!" << endl;
    }
    return 0;
}

One good way to avoid this problem is to type both members of a matching pair of brackets before you fill them in.

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... undeclared

This error is often caused by:

  • Changing the case of a variable after declaring it
  • Forgetting to declare a variable
  • Forgetting to put double quotes around a literal string

For example, the following code:

#include <iostream>
using namespace std;

int main() {
    string message = "Hello world!";
    cout << Message << endl;
    return 0;
}

generates the following error:

fiddle.cpp: In function `int main()':
fiddle.cpp:6: error: `Message' undeclared (first use this
   function)
fiddle.cpp:6: error: (Each undeclared identifier is
   reported only once for each function it appears in.)

The fix is quite simple: pay attention to the case of your identifiers. Remember that C++ is case sensitive.

Another common cause of this error is forgetting to declare a variable. For example, the following code:

#include <iostream>
using namespace std;

int main() {
    cout << message << endl;
    return 0;
}

generates the following error:

fiddle.cpp: In function `int main()':
fiddle.cpp:5: error: `message' undeclared (first use this
   function)
fiddle.cpp:5: error: (Each undeclared identifier is
   reported only once for each function it appears in.)

Fix it by declaring the message variable before using it.

Yet another common cause of this error is forgetting to put double-quotes (") around strings. For example, the following code:

#include <iostream>
using namespace std;

int main() {
    cout << Hello world! << endl;
    return 0;
}

generates the following error:

fiddle.cpp: In function `int main()':
fiddle.cpp:5: error: `Hello' undeclared (first use this
   function)
fiddle.cpp:5: error: (Each undeclared identifier is
   reported only once for each function it appears in.)
fiddle.cpp:5: error: parse error before `!' token

The fix is to put double quotes around the message you want to display:

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undefined reference to ...

This error is often caused by typing the name of a function or variable incorrectly. For example, the following code:

#include <iostream>
using namespace std;

void printMessage();

int main() {
    printMessage();
    return 0;
}

void PrintMessage() {
    cout << "Hello world!" << endl;
}

generates the following, rather confusing, error:

Temp/cchu6zaY.o(.text+0x1f):fiddle.cpp: undefined reference
   to `printMessage()'
collect2: ld returned 1 exit status

The fix, however, is very simple. C++ is case-sensitive. Each time you refer to a function you must use the same case.

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Last Updated: June 21 2005 @17:53:31