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UNSW COMP6771 (Advanced C++) Week 1 Intro

C++ vs C

// helloworld.cpp
#include <iostream>

int main() {
  std::cout << "Hello, world!\n";
  return 0;
}

“\n” vs std::endl

// The following two are equivalent
 
std::cout << "\n" << std::flush;
std::cout << std::endl;

When streaming things to stdout, things are stored in a buffer. things are sent to terminal when the buffer is flushed. std::endl sends a \n and also do a flush. For devices with limited computational power, \n is better as it gives better performance.

Runtime error

#include <string>

int main() {
  std::string s = "";
  s[0];
}

Runtime error is compiler-dependent. This error might crash and also might be ignored.

const

#include <iostream>
#include <vector>

int main() {
    const int i = 0; // i is an int
    i++; // not allowed
    std::cout << i << '\n'; // allowed
    
    const std::vector<int> vec;
    vec[0]; // allowed
    vec[0]++; // not allowed
    vec.push_back(0); // not allowed
}

References

int i = 1;
int j = 2;

int& k = i;
k = j; // This does not make k reference j instead of i. It just changes the value.
std::cout << "i = " << i << ", j = " << j << ", k = " << k << '\n';

Reference is similar to C, but without the need of using -> to access the value. It is like an alias to the original object. Reference is always const and is not null.

References to const

int i = 1;
const int& ref = i;
std::cout << ref << '\n';
i++; // This is fine
std::cout << ref << '\n';
ref++; // This is not

References to literal value

const int &i = 1; // this works
int &i = 1; // this does not

When references to literal value, it must be const.

const int j = 1;
const int& jref = j; // this is allowed
int& ref = j; // not allowed

The value of the object cannot be modified through the const reference but the original object can still be modified. Reference to const must also be const.

Class templates

std::optional<T>, std::vector<T>, std::unordered_map<KeyT, ValueT> for example.

#include <unordered_map>
#include <vector>

// The following items are all function DECLARATIONS

// Not allowed - type templates are not types
std::vector GetVector();

// std::vector<int> and std::vector<double> are valid types.
std::vector<int> GetIntVector();
std::vector<double> GetDoubleVector();

// So is combining types
std::vector<std::unordered_map<int, std::string>> GetVectorOfMaps();

auto

auto i = 0; // i is an int

std::vector<int> fn();
auto j = fn(); // j is std::vector<int>

// Pointers
int i;
const int *const p = i;
auto q = p; // const int*
auto const q = p;

// References
const int &i = 1; // int
auto j = i; // int
const auto k = i; // const int
auto &r = i; // const int&

Compiler will deduct the actual type of auto. Take exactly the type on the right-hand side but strip off the top-level const and &.

Functions

Default function values

string Rgb(short r = 0, short g = 0, short b = 0);
Rgb();// rgb(0, 0, 0);
Rgb(100);// Rgb(100, 0, 0);
Rgb(100, 200);     // Rgb(100, 200, 0)
Rgb(100, , 200);   // error

Default arguments are like Python, but arguments cannot be specified when calling it. And if one parameter uses default argument, all other parameters also need to have a default value. So void test(int a = 0, int b); does not work.

Pass by reference

void swap(int& x, int& y) {
  int tmp;
  tmp = x;
  x = y;
  y = tmp;
}

With the pass by reference, to use it, just need to call it without pointer styles:

swap(i, j);

Pass by reference is useful when:

Pass by reference is not compatible with C as it does not support it.

Lvalue and Rvalue

Function Overloading

void Print(double d);      // (1)
int  Print(std::string s); // (2)
void Print(char c);        // (3)
Print(3.14);               // call (1)
Print("hello World!");     // call (2)
Print('A');                // call (3)

C++ function overloading is very similar to Java function overloading. Return types are ignored when overloading.
Top-level const is ignored but low-level const is not.

// Top-level const ignored
Record Lookup(Phone p);
Record Lookup(const Phone p); // redefinition

// Low-level const not ignored
Record Lookup(Phone &p); (1)
Record Lookup(const Phone &p); (2)

Phone p;
const Phone q;
Lookup(p); // (1)
Lookup(q); // (2)

constexpr

// Beats a #define any day.
constexpr int max_n = 10;

// This can be called at compile time, or at runtime
constexpr int ConstexprFactorial(int n) {
  return n <= 1 ? 1 : n * ConstexprFactorial(n - 1);
}
constexpr int tenfactorial = ConstexprFactorial(10);

// This may not be called at compile time
int Factorial(int n) {
  return n <= 1 ? 1 : n * Factorial(n - 1);
}
// This will fail to compile
constexpr int ninefactorial = Factorial(9);

A variable that can be calculated at compile time is calculated at compile time. A function whose inputs are known at compile time will run at compile time.
As the computations are done at compile time, the run time of the program will be faster and less computation will be needed at run time.

Build Systems (bazel)

// path/to/BUILD

cc_library(
  name = "hello_world",
  srcs = ["hello_world.cpp"],
  hdrs = ["hello_world.h"],
  deps = []
)

cc_library(
  name = "printer",
  srcs = ["printer.cpp"]
  hdrs = ["printer.h"],
  deps = [
    # If it's declared within the same build
    # file, we can skip the directory
    ":hello_world"
  ]
)

Like make, bazel is another open source build system.

build rules