Programming Fundamentals Using C++

Compiler Error Catalogue

Read the FIRST error; the rest are usually echoes · FAQ · Course home

Errors below are shown in g++ style (Clang and MSVC word things differently, but the causes are identical). This catalogue grows as the course grows.

Missing semicolon

error: expected ';' before '}' token

Cause: a statement on the line above the reported line is missing its ;. Fix: look at the previous line; add the semicolon.

Unknown name / not declared

error: 'cout' was not declared in this scope
error: 'sting' was not declared in this scope

Cause: a misspelled name, or a missing #include, or you forgot std::. Fix: check spelling character by character; #include <iostream> for cout/cin; write std::cout (or meet using properly when the course introduces it).

Undeclared (first use in this function) — old compiler style

error: 'x' undeclared (first use in this function)

Cause: using a variable before declaring it, or a typo in the name (count vs cout is the classic). Fix: declare the variable, or fix the spelling.

Unterminated comment / string

error: unterminated comment
warning: missing terminating " character

Cause: /* without its closing */, or a closing quote " forgotten mid-line. Fix: close the comment or the string.

Invalid preprocessing directive

error: invalid preprocessing directive #inclde

Cause: a typo in a # line. Fix: #include <iostream> — note the spelling and the angle brackets.

Uninitialized variable (warning)

warning: 'total' is used uninitialized [-Wuninitialized]

Cause: reading a variable before giving it a value. Fix: initialize it: int total = 0; — the course treats warnings as errors to fix, never to ignore.

Unused variable (warning)

warning: unused variable 'x' [-Wunused-variable]

Cause: declared but never used — often a sign of an unfinished thought. Fix: use it or remove it.

Multiply-defined / redefinition

error: redefinition of 'int main()'

Cause: two main functions in one file (often from merging example files). Fix: one main per program; move the second into a comment or a function.

Divide-style logic errors (no error at all!)

Symptom: the program runs but prints wrong numbers. Cause: this is a logic error — the compiler understood you perfectly; you asked for the wrong thing. Classic first case: integer division (5 / 2 is 2, not 2.5 — Unit 02 explains). Fix: trace with small values; use the debugger; simplify.

Reading error messages: the general recipe

  1. Read the first error and its line number.
  2. The real cause is often on that line or just above.
  3. Fix one thing, recompile — don’t fix five things before rebuilding.
  4. If the message mentions a name, check spelling, #include, and std::.
  5. Still stuck after a few minutes? Find it in the FAQ or report it via CONTRIBUTING.

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