Road to C++ Programmer #1 - Basics

C++

Last Edited: 8/26/2024

The blog post introduces the very basics of C++.

C++ Basics

Before We Start

As I mentioned in the previous article, C++ is an extension of C, and thus many of the syntax and functionalities of C are compatible with C++. In this article on the basics of C++, I will focus on some basic functionalities that were not available in C. If you haven't already, I recommend checking out the C series. The environment setup for C++ is the same as for C, so if you haven't set up your environment yet, refer to the article Road to C Programmer #0 - Setting Up Environment. For C++, we use the file name extension ".cpp" and compile the code with the "g++" command instead of "gcc."

IO Stream

Standard streams are preconnected communication channels for input and output (IO) between a program and its environment. There are three IO connections: standard input (stdin), standard output (stdout), and standard error (stderr). The printf function in C is just a wrapper function that writes formatted strings to the stdout stream. (This explains what the IO monad is for in Haskell.) In C++, we can still use the printf function, but we can also use the following:

#include <iostream>
 
using namespace std;
 
int main () {
 
    cout << "Hello World" << endl;
 
    return 0;
}

using namespace std allows us to access features related to standard streams in the iostream library. (I will cover namespaces in the future.) cout corresponds to console output, which can output values of any type to the console. << is a text insertion operator that can be used to chain operations. endl indicates the end of the sentence, equivalent to \n. You can accept user input as shown below:

#include <iostream>
 
using namespace std;
 
int main () {
 
    float x;
    cout << "Enter a float: ";
    cin >> x;
    cout << "You entered " << x << endl;
    return 0;
}

cin (console input) takes input from the console, ignoring preceding whitespaces, and >> is the stream extraction operator. The trailing whitespaces and text after them are disregarded. If we fail to cast the input to the declared type of the variable, cin outputs 1. Hence, we can guard against the failure like this:

#include <iostream>
 
using namespace std;
 
int main () {
 
    float x;
    cout << "Enter a float: ";
 
    if (cin >> x) {
        cout << "You entered " << x << endl;
    }
    else {
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        cout << "Invalid Input" << endl;
    }
 
    return 0;
}

cin.clear() is used to clear the error state of the input buffer. cin.ignore() ignores the maximum stream size until it encounters \n, so the subsequent cin won't extract invalid input. There are many other functions in the iostream library, but the above are the very basics you must know.

String

In C, a string is just a character array, and we have learned how dangerous strings can be in terms of memory safety. To make the developer experience a little better, C++ has a string library that exposes a string object.

#include <iostream>
#include <string>
 
using namespace std;
 
int main () {
 
    string x = "abcd";
    cout << "x: " << x << endl;
 
    cout << "size: " << x.size() << endl; // size of text = 4
    cout << "length: " << x.length() << endl; // length of text = 4
 
    cout << "x at 3rd index: " << x[3] << endl;
    cout << "x at 3rd index: " << x.at(3) << endl;
 
    cout << "x 3 characters from 1st index: " << x.substr(1, 3) << endl;
 
    x += "efgh";
    cout << "x: " << x << endl;
    x.append("ijkl");
    cout << "x: " << x << endl;
 
    if (x.empty()) {
        cout << "x is empty." << endl;
    } else {
        cout << "x is not empty." << endl;
        x.clear();
        cout << "x is now empty." << endl; 
    }
    
    return 0;
}

The above shows some methods available for the string object. You can obtain the length of the string using size or length (no more sizeof!), access a character or substring using at, [], or substr, and append a string with the + operator or append. You can also check if a string is empty with empty and clear the string using clear. It utilizes OOP to make these helpful functions available in C++. The library also has the following functions to convert types to and from strings:

#include <iostream>
#include <string>
 
using namespace std;
 
int main () {
 
    string numberStr = to_string(3.14);
    double number = stod(numberStr);
 
    return 0;
}

The string library offers a function helpful when taking user input with whitespace in between, getline.

#include <iostream>
#include <string>
 
using namespace std;
 
int main () {
 
    string fullname;
    cout << "Enter your full name: " << endl;
    getline(cin, fullname);
    cout << "Hello, " << fullname << endl;
 
    return 0;
}

If you use cin >> fullname and type in a full name with whitespace, like "Shohei Ohtani," it will ignore "Ohtani" after the whitespace. getline allows the text after the whitespace to be captured.

Exercises

From this article, there will be an exercise section where you can test your understanding of the material introduced in the article. I highly recommend solving these questions by yourself after reading the main part of the article. You can click on each question to see its answer.

Resources

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