Difference Wiki

Definition vs. Declaration: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on April 24, 2024
Definition specifies what a symbol in programming does or represents, while declaration announces the existence of a symbol without defining its implementation.

Key Differences

In programming, definition and declaration are fundamental concepts. A definition is where a variable, function, or class gets its actual implementation or value, specifying what it does or represents. For instance, when a function's code is written, it's defined. On the other hand, a declaration simply announces the existence of an entity in the program but doesn’t provide its complete implementation.
Definition allocates memory for a variable or the body of a function or class. When you define a variable or function, you are telling the compiler what that variable's value is or what the function's code block contains. Declarations, conversely, do not allocate memory for variables (except for external variables) or detail function/class implementations. They only inform the compiler of the type and name of a variable or function.
Every definition is a declaration, but not every declaration is a definition. For example, when a function is declared, its signature is known to the compiler but not what the function does. When the function is defined, the compiler knows what actions the function performs. This distinction is crucial in avoiding linkage errors and ensuring proper program structure.
Definitions ensure that the compiler knows how to implement a function or the value of a variable. Declarations are especially useful in scenarios like header files in C/C++, where they provide a way to reference functions or variables defined in different files. This separation of declaration from definition helps manage large codebases by separating interface from implementation.
Definitions bring declarations to completion by providing the full details needed by the compiler to construct the symbol's implementation. Declarations can be repeated multiple times, but definitions can lead to errors if repeated, as they create ambiguity about which implementation or value to use.

Comparison Chart

Memory Allocation

Allocates memory (for variables, functions, etc.)
Does not allocate memory (except for external variables)

Implementation Details

Provides full implementation details
Only informs about the existence and type


Used once per symbol
Can be used multiple times


Int a = 10; (Variable definition)
Extern int a; (Variable declaration)

In Functions/Classes

Includes the actual code or value
Only includes the signature or prototype

Definition and Declaration Definitions


The action of making something definite and clear.
Her thesis required further definition to be convincing.


A statement of taxable goods or earnings for duty or tax purposes.
He filed his tax declaration promptly.


An exact statement or description of the nature, scope, or meaning of something.
The dictionary provided a clear definition of 'philosophy'.


A formal or explicit statement or announcement.
The government issued a declaration of emergency.


The degree of distinctness in outline of an object or image.
The photo was remarkable for its sharp definition.


The action of declaring, announcing, or stating something clearly.
Her declaration of love was unexpected.


The action of stating the exact meaning of a word or the nature of something.
The lecture began with the definition of key terms.


A clear expression of something in speech or writing.
The manifesto was a bold declaration of the party's principles.


A statement of the exact meaning of a word or the nature or scope of something.
The legal document contained definitions for every technical term used.


An announcement, often one that is formal and public.
The athlete made a declaration about retiring from professional sports.


A statement of the meaning of a word, phrase, or term, as in a dictionary entry.


An explicit, formal announcement, either oral or written.


What happens if you only declare but don't define a function?

The program will compile, but linking will fail due to the missing function definition.

What is a declaration in programming?

It's the announcement of the existence of a variable, function, or class without defining it.

Why are declarations important in programming?

They inform the compiler about the type and existence of an entity without detailing its implementation.

Why are header files full of declarations?

They declare functions and variables so that other files can reference them without needing their definitions.

How does definition contribute to image clarity?

In imaging, definition refers to the clarity and distinctness of outlines in an image.

What is a tax declaration?

It's a statement of income, expenses, or other tax-relevant data filed with tax authorities.

Can a programming entity be declared more than once?

Yes, entities can be declared multiple times, but defined only once.

What is a definition in programming?

It's where variables, functions, or classes get their actual implementation or value.

What is the significance of a declaration in legal terms?

It's a formal statement, often used to assert rights or intentions legally.

How does a definition help in legal contexts?

It provides clear, exact meanings of terms to avoid ambiguity in legal documents.

What error results from multiple definitions of the same entity?

It leads to linkage errors due to ambiguity in implementation.

How does a definition differ from a declaration in terms of memory allocation?

Definitions allocate memory, while declarations do not (except for external variables).

Can a definition exist without a declaration?

No, every definition is inherently a declaration.

How does a declaration function in international relations?

It's a formal statement of policy or position by a country or organization.

What is the role of definition in photography?

It refers to how distinctly an image captures and defines its subject.

Are declarations necessary in all programming languages?

Most languages use declarations, but the specifics vary depending on language design.

What is the purpose of a definition in a dictionary?

It provides the exact meaning and explanation of a word.

What is an example of a declaration in history?

The Declaration of Independence is a famous historical example.

Is definition important in academic writing?

Yes, clear definitions of terms are crucial for clarity and precision in academic writing.

Why might a declaration be used in sports?

Athletes or officials might make declarations about rules, intentions, or retirement.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

Trending Comparisons

Popular Comparisons

New Comparisons