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Debugging Common Errors in C++

~ / Resources / Beginners' / Debugging Common Errors in C++

When trying to find errors in code, a debugger is often used. A debugger allows users to find problems in their code. If the code failed and crashed when running or compiling, a message will be displayed on the debugger showing where and what caused the code to crash.

Code::Blocks is built with a debugger. It should naturally be located on the bottom of the Code::Blocks window. There should be a bar with the name, *Logs & Others* on it. If there is no debugger shown on Code::Blocks, press F2 or go to View > Logs.

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Figure 1. An error being displayed in the Build messages tab. Here, we are told that the error is due to a missing semicolon on line 6.

In the debugger, the *Build messages* tab will display any messages when the computer is building/running a code file. If an error occurs, it will display a message and the line number the error is found in. In case you have trouble understanding what the message means, below are a few errors beginners to that would output a build message,

Error: redeclaration of '_'

  • Did you initialize a variable twice? Two variables should not have the same names.

Error: expected ';' before …​..

  • You simply missed a semicolon on the line right before the line that errored.

Error: '__' was not declared in this scope…​…​.

  • This message is a little vague

  • Is the word that is not declared supposed to be a variable?

    • Is that variable initialized (Check spelling)?

    • Did you initialize a variable inside a loop/if structure and tried to access that variable outside of the loop?

  • Is the word that is not declared supposed to be a command?

    • Did you include both the standard library as well as the library the command is in.

      • Does your file have a “using namespace std;” and an include statement at the beginning of your file?

  • Is the word that is not declared supposed to be a function?

    • Is there a part in your code where a function is used before the function is declared?

      • If a function is placed after the main function, your code cannot call the function inside the main function.

  • Is the word that is not declared supposed to be text?

    • Put quotation marks around them

Fatal error: <name of library>: No such file or directory; …​..

  • Most likely, your file is saved as a C file and not a C++ file. In a C file, many libraries such as string, iostream, and vectors, cannot be used.

  • Did you spell the name of the library correctly?

Error: no match for 'operator>>'…​…​. or Error: no match for 'operator\<<'…​..

  • Did you mix up >> and <<? (<< are for cout statements and >> are for input statements)

Error: invalid operands of types 'const char[ ]' and 'const char[ ]' to

binary…​…​. * Did you try to add two string literals? (text that are surrounded by quotation marks) * Avoid doing that altogether, but if you really need to add them, just omit the *+ sign.

Error: no match for 'operator\<'

  • This applies to every operator

  • What happened was likely an error due to the order the computer performs operations. For example, << is usually executed before a relational operator in an output statement

    • Just add parenthesis and see if it works

Error: invalid operands of types '_' and '_' to 'binary operator%'…​…​..

  • Did you try to perform a modulus operation when one of the values is not an integer/character?

    • If you are trying to mod a floating number, put (int) right in front of the variable to change the floating number into an integer. However, this will cut off any decimals stored in the float. If your code does not crash, but it is not outputting the values that you expect it should output, check the following:

  • Did you use any assignment variables(=) in an if/loop condition when you should have used a relational operator (==)

    • Do not mix = and == up

  • When you initialize a counter variable for your for loop, did you give your counter a value?

    • If not, then the value of the counter will be some garbage number.

  • Are the variables that are sent into a function as parameters the intended variable type?

These are only a few errors that could come up when coding a C++ file. If you are still left with an error with a build message, read through your code once more to see if all of your code makes sense.