Constexpr - Revision history

Rudra at 11:06, 12 November 2018

2018-11-12T11:06:11Z

Rudra: /* Basics */

2018-09-02T04:03:27Z

‎Basics

Rudra at 04:01, 2 September 2018

2018-09-02T04:01:22Z

Rudra: Created page with "`constexpr` is a keyword applied to variables and functions. It's basically the same as the `http://wiki.planetchili.net/index.php?title=Const const`..."

2018-09-02T03:40:39Z

Created page with "<code>constexpr</code> is a keyword applied to variables and functions. It's basically the same as the <code>http://wiki.planetchili.net/index.php?title=Const const</code>..."

New page

<code>constexpr</code> is a keyword applied to variables and functions. It's basically the same as the <code>[[http://wiki.planetchili.net/index.php?title=Const const]]</code> keyword but enforces an additional rule that "The value of this const will be available at compile time"

== Difference between const & constexpr ==
=== Basics ===
In C++ there can be 2 types of constants:
<ol style="list-style-type: decimal;">
<li><p>Whose value is known at compile time.</p>
</li></ol>
<source>
const int x = 50;
const int z = x + 1;
</source>

During compile time, the compiler knows the value of x and z would be 50 and 51 respectively. And the values won’t change throughout the lifetime of the program. So it goes “I’ll just substitute all the references to x and z with 50 and 51, instead of looking up what the values of x and z every time during runtime”.

This allows 50 and 51 to be used as literals (hardcoded values) with all their advantages and at the same time not clutter our code with random numbers that are hard to interpret the meaning of.

<ol start="2" style="list-style-type: decimal;">
<li><p>Whose value is not known at compile time.</p>
</li></ol>
<source>const int y = GenRandomNumber();</source>
In this case the value of y would be impossible to determine at compile time as a we don’t know what random number would be generated during runtime.

NOTE: the variable is still a constant, once a random number is assigned to it the value cannot change.

When the compiler comes across scenario #1 it marks x an z as constant expressions i.e. their value can be evaluated at compile time and in scenario #2 marks y as being a constant but not a constant expression as it can only be evaluated during runtime.

This can be a bit confusing in certain contexts e.g.
<source>
int anArray[x]; // Compiles successfully
int anotherArray[y]; // Fails compilation
</source>
Certain operations require compile-time knowledge of a variable, in this case, anotherArray declaration would throw an error even though y is a const just like x. This is because the value of y cannot be computed until the function GenRandomNum() is run.

Due to this confusion, it is hard to evaluate what constants will be available during compile time, and which won’t like in our example above. As your code base gets bigger this can spiral out of control, as multiple people are working on the same code base.

This is where the keyword <code>constexpr</code> comes in as it tells the compiler that the value of this variable should be deduced at compile time to allow for scenarios such as the one above. If y was declared as <code> constexpr int y = GetRandomNumber()</code> the compiler would throw an error that the value cannot be evaluated at compile time. It clarifies the intended use of the variable.

== Planet Chili Tutorial Video ==
* [[Beginner C++ Game Programming Tutorial 9]]

== Other Tutorials ==

* https://hackernoon.com/a-tour-of-c-17-if-constexpr-3ea62f62ff65

* https://www.geeksforgeeks.org/understanding-constexper-specifier-in-c/

* https://en.cppreference.com/w/cpp/language/constexpr

== See Also ==
* [[Beginner_C++_Game_Programming_Series#Topics_Covered|List of Beginner C++ Topics]]

@@ Line 41: / Line 41: @@
 constexpr int y = 69; // Compiles fine
 </source>
 == Planet Chili Tutorial Video ==

← Older revision		Revision as of 04:03, 2 September 2018
Line 36:		Line 36:

	This is where the keyword <code>constexpr</code> comes in as it tells the compiler that the value of this variable should be deduced at compile time to allow for scenarios such as the one above. If y was declared as <code> constexpr int y = GetRandomNumber()</code> the compiler would throw an error that the value cannot be evaluated at compile time. It clarifies the intended use of the variable.		This is where the keyword <code>constexpr</code> comes in as it tells the compiler that the value of this variable should be deduced at compile time to allow for scenarios such as the one above. If y was declared as <code> constexpr int y = GetRandomNumber()</code> the compiler would throw an error that the value cannot be evaluated at compile time. It clarifies the intended use of the variable.
		+
		+	<source>
		+	constexpr int y = GetRandomNumber(); // error
		+	constexpr int y = 69; // Compiles fine
		+	</source>

	== Planet Chili Tutorial Video ==		== Planet Chili Tutorial Video ==

@@ Line 20: / Line 20: @@
 </li></ol>
 <source>const int y = GenRandomNumber();</source>
-In this case the value of y would be impossible to determine at compile time as a we don’t know what random number would be generated during runtime.
+In this case the value of y would be impossible to determine at compile time as a we don’t know what random number would be generated at runtime.
 NOTE: the variable is still a constant, once a random number is assigned to it the value cannot change.

Constexpr - Revision history

Rudra at 11:06, 12 November 2018

Rudra: /* Basics */

Rudra at 04:01, 2 September 2018

Rudra: Created page with "constexpr is a keyword applied to variables and functions. It's basically the same as the http://wiki.planetchili.net/index.php?title=Const const..."

Rudra: Created page with "`constexpr` is a keyword applied to variables and functions. It's basically the same as the `http://wiki.planetchili.net/index.php?title=Const const`..."