Modern C++
Programming
4. Basic Concepts III
Entities and Control Flow
Federico Busato
2024-03-29
Entities
A C++ program is set of language-specific keywords (for, if, new, true, etc.),
identifiers (symbols for variables, functions, structures, namespaces, etc.), expressions
defined as sequence of operators, and literals (constant value tokens)
C++ Entity
An entity is a value, object, reference, function, enumerator, type, class member, or
template
Identifiers and user-defined operators are the names used to refer to entities
Entities also captures the result(s) of an expression
Preprocessor macros are not C++ entities
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Declaration/Definition
Declaration/Prototype
A declaration (or prototype) introduces an entity with an identifier describing its
type and properties
A declaration is what the compiler and the linker needs to accept references (usage) to
that identifier
Entities can be declared multiple times. All declarations are the same
Definition/Implementation
An entity definition is the implementation of a declaration. It defines the properties
and the behavior of the entity
For each entity, only a single definition is allowed
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Declaration/Definition Function Example
void f(int a, char* b); // function declaration
void f(int a, char*) { // function definition
... // "b" can be omitted if not used
}
void f(int a, char* b); // function declaration
// multiple declarations is valid
f(3, "abc"); // usage
void g(); // function declaration
g(); // linking error "g" is not defined
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Declaration/Definition struct Example
A declaration without a concrete implementation is an incomplete type (as void )
struct A; // declaration 1
struct A; // declaration 2 (ok)
struct B { // declaration and definition
int b;
// A x; // compile error incomplete type
A* y; // ok, pointer to incomplete type
};
struct A { // definition
char c;
}
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Enumerator - enum
Enumerator
An enumerator enum is a data type that groups a set of named integral constants
enum color_t { BLACK, BLUE, GREEN };
color_t color = BLUE;
cout << (color == BLACK); // print false
The problem:
enum color_t { BLACK, BLUE, GREEN };
enum fruit_t { APPLE, CHERRY };
color_t color = BLACK; // int: 0
fruit_t fruit = APPLE; // int: 0
bool b = (color == fruit); // print 'true'!!
// and, most importantly, does the match between a color and
// a fruit make any sense?
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Strongly Typed Enumerator - enum class
enum class (C++11)
enum class (scoped enum) data type is a type safe enumerator that is not implicitly
convertible to int
enum class Color { BLACK, BLUE, GREEN };
enum class Fruit { APPLE, CHERRY };
Color color = Color::BLUE;
Fruit fruit = Fruit::APPLE;
// bool b = (color == fruit) compile error we are trying to match colors with fruits
// BUT, they are different things entirely
// int a1 = Color::GREEN; compile error
// int a2 = Color::RED + Color::GREEN; compile error
int a3 = (int) Color::GREEN; // ok, explicit conversion
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enum/enum class Features
• enum/enum class can be compared
enum class Color { RED, GREEN, BLUE };
cout << (Color::RED < Color::GREEN); // print true
• enum/enum class are automatically enumerated in increasing order
enum class Color { RED, GREEN = -1, BLUE, BLACK };
// (0) (-1) (0) (1)
Color::RED == Color::BLUE; // true
• enum/enum class can contain alias
enum class Device { PC = 0, COMPUTER = 0, PRINTER };
• C++11 enum/enum class allows setting the underlying type
enum class Color : int8_t { RED, GREEN, BLUE };
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enum class Features - C++17
• C++17 enum class supports direct-list-initialization
enum class Color { RED, GREEN, BLUE };
Color a{2}; // ok, equal to Color:BLUE
• C++17 enum/enum class support attributes
enum class Color { RED, GREEN, BLUE [[deprecated]] };
auto x = Color::BLUE; // compiler warning
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enum class Features - C++20
• C++20 allows introducing the enumerator identifiers into the local scope to
decrease the verbosity
enum class Color { RED, GREEN, BLUE };
switch (x) {
using enum Color; // C++20
case RED:
case GREEN:
case BLUE:
}
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enum/enum class - Common Errors
• enum/enum class should be always initialized
enum class Color { RED, GREEN, BLUE };
Color my_color; // "my_color" may be outside RED, GREEN, BLUE!!
• C++17 Cast from out-of-range values respect to the underlying type of
enum/enum class leads to undefined behavior
enum Color : uint8_t { RED, GREEN, BLUE };
Color value = 256; // undefined behavior
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enum/enum class and constexpr
⋆
• C++17 constexpr expressions don’t allow out-of-range values for (only) enum
without explicit underlying type
enum Color { RED };
enum Fruit : int { APPLE };
enum class Device { PC };
// constexpr Color a1 = (Color) -1; compile error
const Color a2 = (Color) -1; // ok
constexpr Fruit a3 = (Fruit) -1; // ok
constexpr Device a4 = (Device) -1; // ok
Construction Rules for enum class Values
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struct 1/2
A struct (structure) aggregates different variables into a single unit
struct A {
int x;
char y;
};
It is possible to declare one or more variables after the definition of a struct
struct A {
int x;
} a, b;
Enumerators can be declared within a struct without a name
struct A {
enum {X, Y}
};
A::X;
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struct 2/2
It is possible to declare a struct in a local scope (with some restrictions), e.g.
function scope
int f() {
struct A {
int x;
} a;
return a.x;
}
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Anonymous and Unnamed struct
⋆
Unnamed struct : a structured without a name, but with an associated type
Anonymous struct : a structured without a name and type
The C++ standard allows unnamed struct but, contrary to C, does not allow
anonymous struct (i.e. without a name)
struct {
int x;
} my_struct; // unnamed struct, ok
struct S {
int x;
struct { int y; }; // anonymous struct, compiler warning with -Wpedantic
}; // -Wpedantic: diagnose use of non-strict ISO C++ extensions
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Bitfield
Bitfield
A bitfield is a variable of a structure with a predefined bit width. A bitfield can hold
bits instead bytes
struct S1 {
int b1 : 10; // range [0, 1023]
int b2 : 10; // range [0, 1023]
int b3 : 8; // range [0, 255]
}; // sizeof(S1): 4 bytes
struct S2 {
int b1 : 10;
int : 0; // reset: force the next field
int b2 : 10; // to start at bit 32
}; // sizeof(S1): 8 bytes
• Useful to compress different values that have specific ranges (e.g. S1)
• ”Free” modulo operation 2
x
(e.g. S1.b1 modulo 2
10
)
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union 1/2
Union
A union is a special data type that allows to store different data types in the same
memory location
• The union is only as big as necessary to hold its largest data member
• The union is a kind of “overlapping” storage
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union 2/2
union A {
int x;
char y;
}; // sizeof(A): 4
A a;
a.x = 1023; // bits: 00..000001111111111
a.y = 0; // bits: 00..000001100000000
cout << a.x; // print 512 + 256 = 768
NOTE: Little-Endian encoding maps the bytes of a value in memory in the reverse order. y
maps to the last byte of x
Contrary to struct , C++ allows anonymous union (i.e. without a name)
C++17 introduces std::variant to represent a type-safe union
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[[deprecated]] Attribute 1/3
C++14 allows to deprecate, namely discourage, use of entities by adding the
[[deprecated]] attribute, optionally with a message
[[deprecated("reason")]] . It applies to:
• Functions
• Variables
• Classes and structures
• Enumerators. Single value enumerator in C++17
• Types
• Namespaces
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[[deprecated]] Attribute 2/3
[[deprecated]] void f() {}
struct [[deprecated]] S1 {};
using MyInt [[deprecated]] = int;
struct S2 {
[[deprecated]] int var = 3;
[[deprecated]] static constexpr int var2 = 4;
};
f(); // warning
S1 s1; // warning
MyInt i; // warning
S2{}.var; // warning
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[[deprecated]] Attribute 3/3
enum [[deprecated]] E { EnumValue };
enum class MyEnum { A, B [[deprecated]] = 42 }; // C++17
namespace [[deprecated("please use my_ns_v2")]] my_ns {
const int x = 5;
}
auto x = EnumValue; // warning
MyEnum::B; // warning
my_ns::x; // warning, "please use my_ns_v2"
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if Statement
The if statement executes the first branch if the specified condition is evaluated to
true , the second branch otherwise
• Short-circuiting:
if (<true expression> r| array[-1] == 0)
... // no error!! even though index is -1
// left-to-right evaluation
• Ternary operator:
<cond> ? <expression1> : <expression2>
<expression1> and <expression2> must return a value of the same or convertible
type
int value = (a == b) ? a : (b == c ? b : 3); // nested
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for and while Loops
• for
for ([init]; [cond]; [increment]) {
...
}
To use when number of iterations is known
• while
while (cond) {
...
}
To use when number of iterations is not known
• do while
do {
...
} while (cond);
To use when number of iterations is not known, but there is at least one iteration
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for Loop Features and Jump Statements
• C++ allows “in loop” definitions:
for (int i = 0, k = 0; i < 10; i++, k += 2)
...
• Infinite loop:
for (;;) // also while(true);
...
• Jump statements (break, continue, return):
for (int i = 0; i < 10; i++) {
if (<condition>)
break; // exit from the loop
if (<condition>)
continue; // continue with a new iteration and exec. i++
return; // exit from the function
}
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Range-based for Loop 1/3
C++11 introduces the range-based for loop to simplify the verbosity of traditional
for loop constructs. They are equivalent to the for loop operating over a range of
values, but safer
The range-based for loop avoids the user to specify start, end, and increment of the
loop
for (int v : { 3, 2, 1 }) // INITIALIZER LIST
cout << v << " "; // print: 3 2 1
int values[] = { 3, 2, 1 };
for (int v : values) // ARRAY OF VALUES
cout << v << " "; // print: 3 2 1
for (auto c : "abcd") // RAW STRING
cout << c << " "; // print: a b c d
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Range-based for Loop ⇝ 2/3
Range-based for loop can be applied in three cases:
• Fixed-size array int array[3] , "abcd"
• Branch Initializer List {1, 2, 3}
• Any object with begin() and end() methods
std::vector vec{1, 2, 3, 4};
for (auto x : vec) {
cout << x << ", ";
// print: "1, 2, 3, 4"
int matrix[2][4];
for (auto& row : matrix) {
for (auto element : row)
cout << "@";
cout << "\n";
}
// print: @@@@
// @@@@
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Range-based for Loop ⇝ 3/3
C++17 extends the concept of range-based loop for structure binding
struct A {
int x;
int y;
};
A array[] = { {1,2}, {5,6}, {7,1} };
for (auto [x1, y1] : array)
cout << x1 << "," << y1 << " "; // print: 1,2 5,6 7,1
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switch 1/2
The switch statement evaluates an expression ( int , char , enum class , enum )
and executes the statement associated with the matching case value
char x = ...
switch (x) {
case 'a': y = 1; break;
default: return -1;
}
return y;
Switch scope:
int x = 1;
switch (1) {
case 0: int x; // nearest scope
case 1: cout << x; // undefined!!
case 2: { int y; } // ok
// case 3: cout << y; // compile error
}
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switch 2/2
Fall-through:
MyEnum x
int y = 0;
switch (x) {
case MyEnum::A: // fall-through
case MyEnum::B: // fall-through
case MyEnum::C: return 0;
default: return -1;
}
C++17 [[fallthrough]] attribute
char x = ...
switch (x) {
case 'a': x++;
[[fallthrough]]; // C++17: avoid warning
case 'b': return 0;
default: return -1;
}
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Control Flow with Initializing Statement
Control flow with initializing statement aims at simplifying complex actions before
the condition evaluation and restrict the scope of a variable which is visible only in the
control flow body
C++17 introduces if statement with initializer
if (int ret = x + y; ret < 10)
cout << ret;
C++17 introduces switch statement with initializer
switch (auto i = f(); x) {
case 1: return i + x;
C++20 introduces range-for loop statement with initializer
for (int i = 0; auto x : {'A', 'B', 'C'})
cout << i++ << ":" << x << " "; // print: 0:A 1:B 2:C
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goto 1/4
When goto could be useful:
bool flag = true;
for (int i = 0; i < N && flag; i++) {
for (int j = 0; j < M && flag; j++) {
if (<condition>)
flag = false;
}
}
become:
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
if (<condition>)
goto LABEL;
}
}
LABEL: ;
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goto 2/4
Best solution:
bool my_function(int M, int M) {
for (int i = 0; i < N; i++) {
for (int j = 0; j < M; j++) {
if (<condition>)
return false;
}
}
return true;
}
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goto 3/4
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goto 4/4
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Avoid Unused Variable Warning 1/3
Most compilers issue a warning when a variable is unused. There are different
situations where a variable is expected to be unused
// EXAMPLE 1: macro dependency
int f(int value) {
int x = value;
# if defined(ENABLE_SQUARE_PATH)
return x * x;
# else
return 0;
# endif
}
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Avoid Unused Variable Warning
⋆
2/3
// EXAMPLE 2: constexpr dependency (MSVC)
template<typename T>
int f(T value) {
if constexpr (sizeof(value) >= 4)
return 1;
else
return 2;
}
// EXAMPLE 3: decltype dependency (MSVC)
template<typename T>
int g(T value) {
using R = decltype(value);
return R{};
}
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Avoid Unused Variable Warning 3/3
There are different ways to solve the problem depending on the standard used
• Before C++17: static cast<void>(var)
• C++17 [[maybe unused]] attribute
• C++26 auto
[[maybe unused]] int x = value;
int y = 3;
static_cast<void>(y);
auto _ = 3;
auto _ = 4; // _ repetition is not an error
void f([[maybe unused]] int x) {}
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