Type Deduction && Initialization of variables

When a new variable is initialized, the compiler can figure out what the type of the variable is automatically by the initializer. For this, it suffices to use auto as the type specifier for the variable:

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int foo = 0; auto bar = foo; // the same as: int bar = foo;

Here, bar is declared as having an auto type; therefore, the type of bar is the type of the value used to initialize it: in this case it uses the type of foo, which is int.

Variables that are not initialized can also make use of type deduction with the decltype specifier:

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int foo = 0; decltype(foo) bar; // the same as: int bar;

Here, bar is declared as having the same type as foo.

In C++, there are three ways to initialize variables. They are all equivalent and are reminiscent of the evolution of the language over the years:

The first one, known as c-like initialization (because it is inherited from the C language), consists of appending an equal sign followed by the value to which the variable is initialized:

type identifier = initial_value; 
For example, to declare a variable of type int called x and initialize it to a value of zero from the same moment it is declared, we can write:

int x = 0;

A second method, known as constructor initialization (introduced by the C++ language), encloses the initial value between parentheses (()):

type identifier (initial_value); 
For example:

int x (0);

Finally, a third method, known as uniform initialization, similar to the above, but using curly braces ({}) instead of parentheses (this was introduced by the revision of the C++ standard, in 2011):

type identifier {initial_value}; 
For example:

int x {0};