C++ Syntax and Program Structure: Statements, Blocks, Namespaces and Headers

Every language has grammar rules. C++'s rules are strict, but they are consistent, and the compiler tells you when you break one. This lesson covers the rules you will use in every program.

Statements end with a semicolon#

A statement is one instruction. In C++, every statement ends with ;:

#include <iostream>

int main() {
    int apples = 5;
    int oranges = 3;
    std::cout << apples + oranges << '\n';
}

8

Line breaks do not end statements; semicolons do. That means one statement can span several lines, which is useful for long expressions:

#include <iostream>

int main() {
    std::cout << "This statement "
              << "continues on "
              << "three lines.\n";
}

This statement continues on three lines.

Blocks use curly braces#

A block groups statements with { and }. Functions, if statements and loops all use blocks. Indentation is not required by the compiler, but it is essential for people reading your code. Use 4 spaces (or your editor's default) consistently:

#include <iostream>

int main() {
    int temperature = 31;
    if (temperature > 30) {
        std::cout << "It's hot.\n";
        std::cout << "Drink water.\n";
    }
}

It's hot.
Drink water.

C++ is case-sensitive#

score, Score and SCORE are three different names. Keywords such as int, if and return are always lowercase.

Identifiers: naming rules#

Names you choose for variables and functions are called identifiers. They:

• may contain letters, digits and underscores (_);
• must not start with a digit (2nd_place is invalid);
• must not be a keyword (int, class, return, …).

Common styles are snake_case (total_price, used by the standard library) and camelCase (totalPrice). Pick one and stay consistent. Avoid names that start with an underscore followed by a capital letter, or that contain double underscores; those are reserved for the compiler.

Namespaces and std::#

A namespace groups related names so they do not clash with names in other code. Everything in the C++ standard library lives in the namespace std, so you write std::cout, std::string and std::vector. The :: is the scope resolution operator: "the cout inside std".

You can create your own namespaces too:

#include <iostream>

namespace shop {
    double tax_rate = 0.18;
}

int main() {
    std::cout << "Tax rate: " << shop::tax_rate << '\n';
}

Tax rate: 0.18

Should you write using namespace std?#

Many tutorials start with using namespace std; so they can write cout instead of std::cout. It works, but it pulls hundreds of names from the standard library into your code, which can clash with your own names. For example, the standard library has std::count, std::size and std::distance, which are also very natural names for variables or functions.

This course writes std:: explicitly, which is what most professional codebases do. If you want shorter names, bring in only what you use:

#include <iostream>
#include <string>

using std::cout;
using std::string;

int main() {
    string city = "Pune";
    cout << city << '\n';
}

Pune

Never put using namespace std; in a header file, because it then affects every file that includes that header.

Header files and #include#

#include copies the contents of a header into your file before compiling. Standard library headers use angle brackets and have no .h extension:

Include a header for everything you use, even if it happens to work without it on your compiler. Your own headers use quotes: #include "shop.h".

Whitespace#

Spaces, tabs and blank lines between tokens are mostly ignored, so these two lines are the same to the compiler:

int total=price*quantity;
int total = price * quantity;

Always write the second one. Readable code is easier to fix.

Common mistakes#

Practice#

#include <iostream>

namespace school {
    int students = 42;
}

int main() {
    std::cout << "Students: " << school::students << '\n';
}

Students: 42

Key takeaway#