Every C++ Feature You Need to Get Started

A Reference Guide for Every Modern C++ Feature

Contents
By Russell Waterhouse · 20 min read · Last updated: November 8, 2023
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EDIT:

I started this blog post in November, then I got sick, and advent of code has started. I’ll continue updating this blog post, but for now, I want what’s already done published more than I want to wait on a complete post. I’ll update this with what I need whenever I get the chance in this chaotic month.

Every C++ Feature You Need to Get Started

I’m going to be learning C++ during advent of code 2023. In advance of this, I’m going to make a quick reference guide for myself for the language. Hopefully, I can get everything that I’ll need in December all in one place.

Disclaimers:

  1. I don’t know C++, and while I did have people who do know C++ proofread this blog post, I don’t claim that I am correct. All errors are my own.
  2. Most of this code won’t compile, has no main, etc. Don’t be sending me “Your code on your blog post would never compile” messages. It’s a reference guide. Imagine that I didn’t miss that semicolon and move on.

Naming Conventions

I’ll be adhering to Google’s naming conventions as much as possible.

Here is a short overview.

Concept Example Explanation
File Name my_file_name.cc All lower case and use - or _. I like _ more.
Type Names MyClass Mixed case, starting with a capital. Classes, structs, enums, etc all follow this.
Variable names (almost everything) my_variable snake_case
Variable names (Data members of classes) my_class_data_member_ Trailing underscore (_)
Variable names (Data members of classes) my_class_data_member_ Trailing underscore (_)
Constant kConstant Use snakeCase and are prepended with k
Functions DeleteTable Mixed case, starting with a capital
Namespace names web_search::search snake case
Enums kOk Same as constants
Macro names #define DAYS_IN_WEEK 7 screaming snake case

Primitive Types

  • int
  • char
  • bool
  • float
  • double
  • void
  • wchar_t (for unicode support)
int foo = 10;
char bar = 'c';
bool baz = true;
float quz = 10.1;
double quuz = 10.00000001;

Operators

1 + 1 // addition
1 - 1 // subtraction
10 % 2 // modulo
true && false // logical AND
true || false // logical OR
0b00000001 & 0b00000001 // bitwise AND
0b00000001 | 0b00000001 // bitwise OR
3 > 2 // greater than
2 < 3 // Less than
3 >= 2 // greater than or equal to
2 <= 3 // Less than or equal to

NULL

  • NULL is a macro for null pointers (pointers that point nowhere).
  • nullptr is the pointer literal which specifies a null pointer value (introduced in C++11).
  • void is a keyword that represents a lack of type or empty type.

Uniform Initialization

This is a slick way to to say “give me a sane default variable of X type” where X type decides what that sane default is. For int, it’s 0, for instance.

int i {}; // i is 0
int j {999}; // j is 999
float k {10.2}; // k is 10.2

References

Like pointers but more strict.
Denoted with &

int foo = 10;
int& bar = {foo}; // uniform initialization again
// both foo and bar now point to the same variable
foo = 20; //foo and bar are both 20
bar = 30; //foo and bar are both 30

Const

Used to signify that this variable does not and can not change.

const int foo = 5; //foo is always 5

// This function doesn't take a copy, but doesn't modify the parameter
int add10(const int& bar) {
    baz = bar + 10;
    return baz;
}

Conditionals

  • if
  • else
  • else if
  • switch (example below
switch (day_of_week) {
    case 1: 
        std::cout << "I Hate Mondays" << std::endl;
        break;
    case 2: 
        std::cout << "I Hate Tuesdays" << std::endl;
        break;
    case 3: 
        std::cout << "I Hate Wednesdays" << std::endl;
        break;
    case 4: 
        std::cout << "I Hate Thursdays" << std::endl;
        break;
    case 5: 
        std::cout << "I Love Fridays" << std::endl;
        break;
    case 6: 
        std::cout << "I Love Saturdays" << std::endl;
        break;
    case 7: 
        std::cout << "I Love Sundays" << std::endl;
        break;
}

Loops

while loops:

int i = 0;
while (i < 10) {
    std::cout << "Another one" << std::endl;
    i++;
}

do while loops:

int i = 0;
do {
    std::cout << "Another one" << std::endl;
    i++;
} while (i < 10);

for loops:

for (int i = 0; i < 99; i++) {
    std::cout << "Another one" << std::endl;
}

for-each loops:

Vector<int> v = {0, 1, 2, 3}
for(int number: v) {
    std::cout << number << std::endl;
}

break and continue offer more control of iteration in loops.

Common Data Structures

This entire section is basically just the codecademy cheat sheet. All I did was throw it in a table and exclude arrays because why would I use one of those while learning C++ when Vectors Exist?

Data Structure #include Library Quick Explanation
Vecor #include <vector> Dynamically sized arrays
Stacks #include <stack> Stacks
Queues #include <queue> Queues
Sets #include <set> Set
Hash Map #include <map> Key Value Pairs

Structs

These should be used when you have simple data and don’t want the features that classes provide. Here’s an example:

struct Location {
    double latitude {0};
    double longitude {0};
}

int main() {
    Location foo {};
    Location bar {100.0. 100.0};
    return 0;
}

Unions

I read a few people on the internet who had the opinion that we shouldn’t use unions in modern C++, so probably just don’t. Nobody on the internet has ever been wrong.

Enums

I love enums.

enum class WeekDay {
    kMonday,
    kTuesday,
    kWednesday,
    kThursday,
    kFriday,
    kSaturday,
    kSunday
};

Functions

Functions take parameters and return values. Default is pass by value.

int printInt(int i) {
    //i is a copy of whatever was passed in
    std::cout << i << std::endl;
    return 0;
}

int printReference(int& j) {
    // j is a reference to the original variable
    // modifying j would also modify the original variable
    std::cout << j << std::endl;
    return 0;
}

int main() {
    int x {99};
    int& y {x};
    printInt(x);
    printReference(y);
    return 0;
}

Error Handling

C++ is a really fun language, it has both c-style segfaults and java-style exception handling.

Throwing Exceptions

#include <iostream>
#include <stdexcept>

int main() {
    throw std::runtime_error("This is a runtime error");
    return -1;
}

Catching Exceptions

#include <iostream>
#include <stdexcept>

int main() {
    try {
        throw std::runtime_error("This is a runtime error");
    } catch (const std::exception& e) {
        std::cerr << "Caught exception: " << e.what() << std::endl;
    }

    return 0;
}

Debugging Segfaults

  1. Compile your program with debug information (the -g flag)
  2. Run your program with gdb (gdb ./program_name)
  3. Examine the output. Example:

Program that could segfault in several places:

// DON'T WRITE ANY CODE LIKE THIS, 
// IT'S LITERALLY ALL UNSAFE
#include <iostream>
#include <vector>

int& CreateSegfault() {
    int x = 4;
    int& y = {x};
    return y;
}

int main() {
    int& a = CreateSegfault();
    int b = 99;
    int c = a + b;
    a++;
    c++;

    std::vector<int> j = {1, 2, 3};
    int& k = j.at(1);
    j.clear();
    k++;

    int& m = *(int*)NULL;
    m++;
    return 0;
}

As an aside, isn’t it crazy that on my machine that compiles without any warnings?
I think that if C++ cannot fix that, Rust will win eventually. I also think that eventually is probably 10 years away.

Output when run:

./segfault
Segmentation fault (core dumped)

GDB output:

gdb ./segfault
GNU gdb (GDB) Fedora Linux 13.2-6.fc38
Copyright (C) 2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./segfault...
(gdb) run
Starting program: /home/russ/repos/programming-tools/tools/debuggers/gdb/segfault

This GDB supports auto-downloading debuginfo from the following URLs:
  <https://debuginfod.fedoraproject.org/>
Enable debuginfod for this session? (y or [n]) n
Debuginfod has been disabled.
To make this setting permanent, add 'set debuginfod enabled off' to .gdbinit.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
0x000000000040129b in main () at segfaults.cc:23
23          m++;

We can see clearly here that we crash on line 23 when incrementing the m variable.

Preventing Errors

These aren’t anything that C++ enforces, I’m just going to quickly describe a holistic approach I’m taking here to make sure that I limit the number of errors that I create.

  • Use smart pointers or references instead of raw pointers.
  • Returning std::optional<> when I might return a value.
  • Limit the use of NULL, void, and null_ptr.
  • Avoid macros.
  • Return status codes from functions that might have problems instead of throwing exceptions.
  • Not doing any weird typecasting, have safe functions for converting types.
  • Using for-each loops when iterating through a data structure.
  • generous use of const and only mutating data when necessary.
  • Keep things as simple as possible.

I do the above to limit the following class of bugs:

  • Memory related problems(segfaults, memory leaks, use after free, etc.).
  • NullPointerException.
  • Macro confusion.
  • Typecasting errors
  • Off by 1 errors
  • Side effect errors (Haskell programmers know what I’m talking about)

Classes

Classic object oriented programming stuff here.

class Vehicle{
public: 
    Vehicle(String name){
        //constructor things
    }
    ~Vehicle() {
        //destructor things
    }

private: 
    void funny() {
        std::cout << "beep boop";
    }
}

Objects

There are a few ways to create objects.

Stack Allocated Objects (Autumatically Managed)

Do this when at all possible.

#include <vector>
#include <memory>
int main(){
    Vector<int> v {10, 20, 30};
    int i = 10;
}

Heap Allocated Objects (Manually Managed)

You probably don’t want to do this, see the section on smart pointers below. That being said, here’s how you would if you wanted to.

#include <vector>
#include <memory>
int main(){
    unique_ptr<Vector<int>> V = std::make_unique();
    unique_ptr<int> V = std::make_unique();
    shared_ptr<String> = std::make_shared();
}

Instance Variables

Declaration:

class MyClass {
public:
    int public_var_1;
    double public_var_2;
private:
    int private_var_1;
    double private_var_2;
};

default initialization:

class MyClass {
public:
    int public_var_1 = 987;
    double public_var_2 = 123.45;
private:
    int private_var_1 = 123;
    double private_var_2 = 987.65;
};

access:

int main() {
    MyClass foo;
    foo.public_var_1 = 10;
    foo.public_var_2 = 20.0;
    foo.private_var_1 = 30; // This will not compile
    foo.private_var_2 = 40.0; // This will not compile
    return 0;
}

Methods

Just declare a function in the class.

class MyClass {
public:
    void myMethod() {
        std::cout << "Hello World" << std::endl;
    }
};

int main() {
    MyClass foo;
    foo.myMethod();
    return 0;
}

Inheritance

class Base {
public:
    void baseMethod() {
        std::cout << "Base Method" << std::endl;
    }
};

class Derived : public Base {
public:
    void derivedMethod() {
        std::cout << "Derived Method" << std::endl;
    }
};

int main() {
    Derived foo;
    foo.baseMethod();
    foo.derivedMethod();
    return 0;
}

Pointer

Don’t use those, use References or Smart Pointers.

Smart Pointers

Unique Pointers

Added in C++ 11, unique pointers are the sole owner of their data, and when they go out of scope they free the memory that they point to. Initialization, freeing, moving

#include <stdmemory>

int main() {
    std::unique_ptr<int> foo = std::make_unique(1);
    std::unique_ptr<int> bar = std::make_unique(2);

    foo = std::move(bar); // 1 is freed, bar is now a nullptr, foo points to 2
    std::unique_ptr<int> baz = std::unique_ptr<int>(std::move(foo)); // foo is nullptr, baz points to 2
    baz.reset(); // baz is nullptr, 2 is freed.
    return 0;
    // No matter what happened before, whether there was exceptions or not,
    // when foo and bar go out of scope, both 1 and 2 will have been freed.
}

Of note, there is no copy, so in the example above you couldn’t have called

foo = bar;

That simply wouldn’t compile.

Shared Pointers

Shared pointers are pointers to a shared object, tracked through reference counting. When the reference counter reaches 0, the memory is freed.

It has more overhead than unique pointer, so unique pointer should be preferred.

Initialization, sharing, and resetting:

#include <memory>

int main() {
    std::shared_ptr<int> foo = std::make_shared<int>(1);
    std::shared_ptr<int> bar = foo; // Both foo and bar now share ownership of the integer with value 1

    // Modifying shared pointers
    *foo = 42; // Changes the value to 42 through either foo or bar

    // Resetting shared pointers
    foo.reset(); // Decrements the reference count; memory is not freed since bar still owns it
    bar.reset(); // Reference count reaches zero; memory is freed

    return 0;
    // No matter what happened before, when foo and bar go out of scope,
    // the dynamically allocated memory will be freed only when the reference count drops to zero.
}

In the shared pointers example above, both foo and bar share ownership of the dynamically allocated integer. Changes made through one shared pointer are reflected in the other. Unlike unique pointers, shared pointers support assignment and copying, as demonstrated by the assignment of bar to foo. The memory is freed only when the last shared pointer pointing to it is reset or goes out of scope.

#include <stdmemory>

int main() {
    std::shared_ptr<int> foo = std::make_shared(1);
    std::shared_ptr<int> bar = std::make_shared(2);


    return 0;
}

Macros

I’m not going to define macros. You probably shouldn’t either.

Modules

Program Arguments

In C++, you access program arguments the same way that you do in C.

Your main function gets an int argc and a char* argv[]. These c-style string arguments can be converted to C++ strings by doing the following:

std::string arg = argv[index];

Program IO

Input from a user (stdin)

string s;
std::cin >> s
std::cout << "The user said: " << s << std::endl;

File IO

#include <fstream>
#include <iostream>
#include <string>

int main() {
    std::ofstream file("example.txt");
    file << "Hello, World!";
    file.close();

    std::ifstream read_file("example.txt");
    std::string line;
    while (std::getline(read_file, line)) {
        std::cout << line << std::endl;
    }
    read_file.close();

    return 0;
}

Functions

Functions are declared in the following way:

int add(int a, int b) {
    return a + b;
}

Compiler

There are many C++ compilers, but I’ll be using g++ because it’s already on my machine (Fedora 38 at the time of writing).
To compile with g++, just use the following command:

g++ -Wall -std=c++20 -o <output_binary_name> <input_file.cc>

In order, the arguments mean the following:

  • -Wall - Print all warnings
  • -std=c++20 - Use the C++ 20 standard (the most recent one at the time of writing)
  • -o name for the output file, default is a.out

Debugger

For a debugger, I don’t think I can do better than GDB, the GNU Debugger. I’ll be using that while learning C++.

Language Server

For a language server, I’m going to be using clangd

In the little experiments I’ve been doing while writing this blog post, it’s been more than sufficient for my needs.

Test Framework

I’ll probably be using gtest, Google’s C++ testing framework. Here’s an example gpt generated about using it:

#include <gtest/gtest.h>

int add(int a, int b) {
    return a + b;
}

TEST(AdditionTest, HandlesPositiveInput) {
    EXPECT_EQ(3, add(1, 2));
}

TEST(AdditionTest, HandlesNegativeInput) {
    EXPECT_EQ(-1, add(-1, 0));
}

// More tests...

Finally, compile your main file with gtest linked, run your code, and watch the tests pass.

#include <gtest/gtest.h>

// ... (your test cases)

int main(int argc, char **argv) {
    ::testing::InitGoogleTest(&argc, argv);
    return RUN_ALL_TESTS();
}

Package Manager

There are many C++ Packages, but I’ll be using the most popular, which is Conan.

I’ll be using this tutorial by ForgottenUmbrella on GitHub to set it up and get using it

Packages

I’m not going to list every popular package on Conan’s center, but I will highlight a few that I might use very soon.

  • OpenSSL: Needs no introduction
  • fmt: A safe and fast alternative to printf and IOStreams
  • gtest: Google’s C++ testing framework
  • nlohmann_json: C++ json parser
  • spdlog: C++ Logging library
  • libbacktrace: Can be linked into a C++ program to produce symbolic backtraces.
#CPP