Noob question about integer literals and suffixes

[chango]

First I apologize for my bad english.

When do you use suffixes or why would I use them if I can specify from the beggining the type of the variable or that is a signed or unsigned int

why would I use for example

int x = 75u;

if I can use

unsigned int x = 75;

reference: http://www.cplusplus.com/doc/tutorial/constants/

[MiiNiPaa]

In such assignments it is mostly useless. But still consider following:
auto x = 75u; //u type is unsigned int

It is useful when you have literal in expression:

unsigned shift = 35;

//Incorrect, does not do what you want
unsigned long long mask = 1 << shift; 

//Correct
unsigned long long mask = 1ull << shift; 

Additionally with UDL you may do things with literals you could not before:

//t represents duration of 3 milliseconds
//f represents duration of 100 nanoseconds
auto t = 3ms;
auto f = 100ns;

//Compare with
auto t = std::chrono::seconds(3);

//OK, sleep for 7 second
std::this_thread::sleep_for(5s);

//Not OK, 7 what? seconds, milliseconds, ticks?
std::this_thread::sleep_for(7);

//OK 7 kilometers are not equal to 7 miles
assert(7_km != 7_m);

[JLBorges]

In most situations, we do not need to use integer-suffixes for our integer literals.

int x = 75u;  // type of 'x' is 'signed int', with or without the suffix on the literal

unsigned int y = -75LL; // type of 'y' is 'unsigned int' 

The tutorial is not precise where it says: "By default, integer literals are of type int".

The type of a decimal integer literal without an integer-suffix is the first of
int, long or long long
in which the value of the literal can be represented.

auto a = 1234 ; // type of 'a' is 'signed int'

auto b = 1234567890123456 ; // type of 'b' is 'long long' (on typical implementations) 

The type of a binary, octal, or hexadecimal integer literal without an integer-suffix is the first of
int, unsigned int, long, unsigned long, long long, unsigned long long
in which the value of the literal can be represented.