Stack too deep

The “stack too deep” error is a common issue in Solidity, the primary programming language for Ethereum smart contracts. This error occurs when a function has too many local variables, causing the compiler to run out of available stack space.
The Ethereum Virtual Machine has a limited stack size, typically allowing for up to 16 local variables in a function. To mitigate this error, one effective technique is to use an extra set of curly braces {} to create a new scope within the function.
Here’s how this works and why it’s effective:

Creating a new scope:* By adding an extra set of curly braces within your function, you create a new scope for variables.

Variable lifecycle: Variables declared within this new scope are only alive for the duration of that scope. Once the code execution leaves the scope, these variables are destroyed, freeing up stack space.

Reusing stack slots:
The EVM can reuse the stack slots occupied by these scoped variables after they’re destroyed, effectively allowing you to have more than 16 variables in total, just not all at the same time.

Here’s an example of how to apply this technique:

function complexFunction() public {
    // Some variables here

    {
        // New scope
        uint256 tempVar1 = someCalculation();
        uint256 tempVar2 = anotherCalculation();
        // Use tempVar1 and tempVar2
    }

    // tempVar1 and tempVar2 no longer exist here, freeing up stack space

    {
        // Another new scope
        uint256 tempVar3 = yetAnotherCalculation();
        // Use tempVar3
    }

    // Continue with the rest of the function
}

This technique is particularly useful when you have variables that are only needed for a specific part of your function. By scoping them appropriately, you can manage your stack usage more efficiently.

Other strategies to mitigate “stack too deep” errors include:

• Refactoring large functions into smaller ones
• Using structs to group related variables
• Storing data in storage instead of memory for complex operations
• Optimizing variable declarations and reusing variables when possible