Tcl variable Command

last modified April 3, 2025

The Tcl variable command declares variables in namespaces and procedures. It ensures variables exist in the current scope before use. This prevents potential issues with variable resolution.

Basic Definition

The variable command creates one or more variables in the current namespace. It's particularly important for procedures that need to access global or namespace variables.

Syntax: variable ?name value?.... It can declare multiple variables at once. Without a value, it just declares the variable's existence.

Basic Variable Declaration

This shows the simplest usage of variable to declare a variable.

basic_variable.tcl

variable name "John Doe"
puts $name

This declares a variable name with the value "John Doe". The puts command then prints the variable's value. This is similar to set but with namespace awareness.

Global Variable Access

The variable command is essential for accessing global variables inside procedures.

variable_global.tcl

set ::counter 0

proc increment {} {
    variable ::counter
    incr ::counter
}

increment
puts $::counter

Here we declare a global variable ::counter and access it inside the increment procedure using variable. This ensures proper variable resolution.

Namespace Variables

variable is crucial for working with namespace variables.

variable_namespace.tcl

namespace eval math {
    variable PI 3.14159
    
    proc area {radius} {
        variable PI
        expr {$PI * $radius * $radius}
    }
}

puts [math::area 5]

This creates a namespace math with a constant PI. The area procedure accesses PI using variable. This maintains proper namespace encapsulation.

Multiple Variable Declaration

variable can declare multiple variables in one command.

variable_multiple.tcl

namespace eval config {
    variable width 800 height 600 title "App"
    
    proc show {} {
        variable width height title
        puts "Config: $width x $height, $title"
    }
}

config::show

This declares three variables in the config namespace at once. The show procedure accesses them using a single variable command. This is more efficient than separate declarations.

Variable Without Initialization

variable can declare a variable without assigning a value.

variable_declare.tcl

proc process {} {
    variable result
    set result "Operation completed"
    return $result
}

puts [process]

Here we declare result without initialization inside a procedure. The variable is later assigned a value with set. This pattern is useful for procedure-local variables.

Combining with upvar

variable can be combined with upvar for advanced variable handling.

variable_upvar.tcl

namespace eval outer {
    variable data "Important information"
    
    proc access {} {
        variable data
        upvar 1 data localdata
        puts "Accessed: $localdata"
    }
}

outer::access

This demonstrates combining variable with upvar to create an alias to a namespace variable. The access procedure can work with the variable through a local name.

Best Practices

Namespace use: Always use variable for namespace variables.
Global access: Prefer variable over global for globals.
Early declaration: Declare variables at procedure start.
Explicit names: Use fully qualified names when needed.
Documentation: Comment namespace variables for clarity.

This tutorial covered the Tcl variable command with practical examples showing its usage in different scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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