FAQ¶

Short answers to the questions that come up most in a first C++ course. Each one points to where the topic is covered in full, and to a longer discussion online where that helps.

Setting up and tools¶

Where should I put my project, and why does it matter? Use a short, plain path near the drive root (C:\dev\…) — no spaces, no Norwegian letters, and not inside a cloud-synced folder. Odd paths cause baffling build errors. See Computer Basics.

I already know some Python — what is different in C++? A lot, underneath: static types, value semantics (b = a copies), manual scope and lifetimes, integer division. See Coming from Python.

Should I let an AI write my C++? As a tutor and a reviewer, yes; as a replacement for understanding, no — they are confidently wrong often enough that you must check everything. See Using AI for Coding.

Why split code into a header and a .cpp, instead of #include-ing a .cpp file? A header shares declarations; including a .cpp copies its definitions into every file, causing "multiple definition" linker errors. See Classes. (More: Stack Overflow.)

#include <foo> versus #include "foo"? Angle brackets for standard and library headers; quotes for your own files. See Basic Structure. (More: Stack Overflow.)

Why the #ifndef/#define (or #pragma once) at the top of headers? They stop a header being pasted into one file twice, which would cause "redefinition" errors. #pragma once is the modern one-liner. See Reading Compiler Errors. (More: Stack Overflow.)

When something goes wrong¶

My program will not compile — what does the error mean? Fix the first error first (the later ones are often fallout), starting at the file:line it names. See Reading Compiler Errors.

It compiles for me but not for my partner (or on another OS) — why? C++ is compiled per platform, and the same source is not guaranteed to compile on every compiler. Usual culprits: a missing #include you got away with, a feature too new for their compiler, word operators like or, M_PI, or assuming a type's size. See Portability.

It compiles and runs, but does the wrong thing — now what? Use the debugger: set a breakpoint, step through, and watch the variables until what is happening and what you expected diverge. See Using a Debugger.

Why does if (x = 5) always run? = assigns; you meant ==, which compares. Turn warnings on and the compiler will flag it. See Operators and Expressions.

Language basics¶

Why does 10 / 3 give 3, not 3.33? Integer division throws the remainder away. Make one side a double (10.0 / 3) for a decimal result. See Operators and Expressions.

std::endl or "\n" — which should I use? Both end the line; std::endl also flushes the stream, which is slower. Prefer "\n". See IO & Streams. (More: Stack Overflow.)

Do I really need braces {} on a one-line if? Yes — always brace. It is one extra line and removes a whole class of bugs when someone later adds a second statement. See Control Statements. (More: Stack Overflow.)

Why would I initialise with curly braces, int x{5}? Mostly for safety: brace initialisation refuses "narrowing" conversions that silently lose data — int x{3.7} will not compile, whereas int x = 3.7; quietly truncates to 3. This book uses plain = for everyday initialisation and reaches for {} when rejecting narrowing matters. See Variables and Basic Types. (More: Stack Overflow.)

What does explicit mean on a constructor? It stops that constructor being used for silent conversions. A one-argument constructor like Motor(int) normally lets the compiler turn a stray int into a Motor on its own; explicit switches that off, so the conversion happens only when you ask for it. Mark single-argument constructors explicit unless you want the conversion. See Classes. (More: Stack Overflow.)

Why is using namespace std; discouraged? It dumps every standard-library name into your scope, inviting clashes and ambiguity. Write the std:: prefix instead. See C++ Standard Library. (More: Stack Overflow.)

What does std:: mean, and why is it everywhere? std is the namespace that holds the standard library; std::cout means "cout, from std." See C++ Standard Library.

Why isn't 0.1 + 0.2 exactly 0.3? Floating-point numbers are approximations, so tiny errors creep in — never compare them with ==. See Floating-Point Pitfalls.

What is a lambda? A small, unnamed function written inline, usually handed to an algorithm. See Lambda Expressions. (More: Stack Overflow.)

Pointers, references, and memory¶

What is the difference between a pointer and a reference? A reference is a permanent alias for one variable; a pointer is a reseatable address that can be null. Prefer a reference unless you need null or reassignment. See Values, References & Pointers. (More: Stack Overflow.)

When do I use . versus ->? . on an object; -> through a pointer to one (p->x is shorthand for (*p).x). See Values, References & Pointers.

Why pass by const&? It avoids copying a large object while promising not to change it — the standard way to pass anything bigger than a number. See Values, References & Pointers.

What is const-correctness? The habit of marking everything that does not change as const: member functions that only observe (read() const), parameters you only read (const T&), and locals you never reassign. It matters because a const object can call only const member functions — so consistent const lets you pass your own types by const& and lets the compiler catch accidental modifications. See Classes and Values, References & Pointers.

Why should I avoid raw new and delete? They are easy to leak or double-free. Let std::vector, std::string, and smart pointers own memory for you (that is RAII). See Memory Management. (More: Stack Overflow.)

Which cast should I use? Avoid the old C-style cast (int)x — it silently does whatever it takes to force the conversion, which hides mistakes, and it is impossible to search for. Use a named cast instead: almost always static_cast; dynamic_cast, const_cast, and reinterpret_cast are rare and each needs a specific reason. See Operators and Expressions. (More: Stack Overflow.)