Errors

Errors

Errors are bugs in your code, and C++ lets you find two very different kinds of them. Knowing which is which tells you whether the compiler or your own eyes should be looking for it.

Syntax errors

A syntax error is a grammar mistake: you wrote something C++ simply refuses to read. The compiler catches these for you before the program even runs.

int main() {
    cout << "Hello World;   // missing closing quote
    return 0               // missing semicolon
}

That file will not compile. The compiler stops and points you at the broken line, so syntax errors are usually the easiest kind to fix.

Semantic errors

A semantic error means the code is legal C++, but it does the wrong thing. The compiler cannot catch it - the program runs fine, it just produces a wrong result.

int x = 5;
x = x - 1;   // you meant to add 1
cout << x;   // prints 4 instead of 6

No warning, no crash: the answer is simply wrong. You asked for something the computer carried out exactly - your mistake, not the compiler's.

Compiler catches, humans catch

• The compiler finds syntax errors automatically.
• Semantic errors are logical mistakes you must find by testing and debugging.

int main() {
    int gates = 10;
    if (gates - 1) {   // wrong comparison, always true
        cout << "always runs";
    }
    return 0;
}

gates - 1 computes a number instead of testing a condition. Small mental slips like this are semantic errors that slip right past the compiler.

TL;DR

• Errors come in two flavors: syntax and semantic.
• Syntax errors are grammar mistakes the compiler catches.
• Semantic errors compile fine but give wrong results.
• Assignment vs comparison is the classic sneaky semantic bug.
• The compiler is your friend for syntax; you must hunt semantic bugs yourself.