Type Casting (static_cast & more)

Sometimes a value is the wrong type and you need to convert it deliberately. C++ gives you four named casts — static_cast , dynamic_cast , const_cast , and reinterpret_cast — each with a clear purpose. This lesson teaches you which to reach for, why they beat C-style (int) casts, and the truncation traps to watch for.

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Think of the four casts as four different adapters . static_cast is a sensible travel adapter — it connects compatible plugs (an int into a double socket). dynamic_cast is a security guard who checks ID at the door and turns you away if you're not really who you claim to be. const_cast is a master key that removes a "do not touch" sticker — powerful but easy to misuse. reinterpret_cast is a crowbar that forces two unrelated things together by brute reinterpretation of their bits. You reach for the gentlest tool that does the job — almost always static_cast .

1. static_cast : the workhorse

This is the cast you'll use 95% of the time. It converts between related types: int ↔ double , char ↔ its numeric code, and pointers within a hierarchy. The classic use is breaking the integer-division trap by casting one operand to double .

2. Truncation vs Rounding

Casting a double to int truncates toward zero — it chops the fraction, it does not round. If you want the nearest whole number, round first with std::round (or use std::lround , which returns an integer directly).

3. dynamic_cast : safe downcasting

When you have a base-class pointer and need to know the real derived type, dynamic_cast checks at runtime. It returns nullptr for pointers (or throws std::bad_cast for references) when the object isn't the type you asked for. It requires the base class to be polymorphic — at least one virtual function.

4. const_cast : removing const (carefully)

const_cast adds or removes const . Its main legitimate use is calling an older API that forgot to mark a read-only parameter const . Never use it to actually modify an object that is genuinely const — that's undefined behavior.

5. reinterpret_cast : raw bits (last resort)

This is the most dangerous cast. It views the same bytes through a completely different type — useful for low-level work like inspecting a value's individual bytes or treating a pointer as an integer address, but non-portable and easy to misuse. Reach for it only when nothing else works.

Your turn. Cast one operand so the percentage keeps its decimals:

These lines round a price to the nearest cent-free whole number and print it. Reorder so the program prints 4 .

Why: must precede any use of round . Inside main , declare price first, then round(3.6) gives 4.0 and static_cast<int> converts that cleanly to 4 (rounding before casting avoids truncating 3.6 down to 3 ). Then print and close the function.

Predict the output before revealing each answer.

2 — casting a double to int truncates toward zero; it does not round to 3.

a — 97 is the ASCII code for lowercase 'a', so casting the int to char prints that character.

2.5 — casting one operand to double makes the whole division floating-point.

Read points earned and possible, compute a rounded percentage with a cast, and map it to a letter grade. Match the expected output in the comments.

Practice quiz

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