Kimi | Function Pointer Struct Size Bloat

For some reason when I assign a function to a struct, my compiled binary size jumps up 0.5kb. I know it is the struct causing this because I can use the function in other parts of my code without it jumping up. The reason I am storing it in a struct anyways is because I need to iterate over some data & find the relevant function to act on that data. Checking every possibility & calling the specific function name from there doesn't sound fun. I wouldn't mind the 0.5k if it was just that, but I have a ton of little modules each with their own struct instance holding their own separate function & it stacks up quick. Here is one of my modules so I can show an example of what EXACTLY I am doing that is causing this behavior: ``` \#pragma once \#include "../Common.hpp" \#include "../ScopeState.hpp" void INST\_End\_exec(const Instruction& inst, const InstToken& token, ScopeState& state, const std::vector<std::string>& args, const std::string& symbol) { return; } Instruction INST\_End { 0, 0, //INST\_End\_exec, }; ```

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Looking at your code, I see the issue. You're storing a function pointer inside a global struct instance, and that struct is being placed in the data section (.data or .rodata) of your binary — even though the function pointer itself is a compile-time constant.

Here's what's happening and how to fix it.

The Problem

When you write:

cpp

Instruction INST_End {
    0,
    0,
    //INST_End_exec,  // commented out
};

Your Instruction struct likely has a function pointer field (the third member). When you uncomment it:

cpp

Instruction INST_End {
    0,
    0,
    INST_End_exec,
};

The compiler emits this global variable into a writable data section (.data) because function pointers in global mutable structs are treated as runtime-initialized data. Even though the value is known at compile time, the compiler often doesn't place it in .rodata (read-only data) because the struct itself is mutable.

The 0.5KB jump isn't the pointer itself (that's 4–8 bytes). It's likely:

Padding/alignment forcing the struct to be larger than expected
The struct being placed in .data instead of .bss or being optimized away entirely
The linker now needing to emit relocation entries for the function pointer, since the actual address of INST_End_exec isn't known until link time

The Fix: Make It `constexpr` or `const`

The cleanest solution is to make the struct constexpr (or at least const), which allows the compiler to place it in .rodata and potentially inline/optimize it entirely:

cpp

struct Instruction {
    int opcode;
    int arg_count;
    void (*exec)(const Instruction&, const InstToken&, ScopeState&, 
                 const std::vector<std::string>&, const std::string&);
};

// Make it constexpr so it lives in .rodata, not .data
constexpr Instruction INST_End {
    0,
    0,
    INST_End_exec,
};

If std::vector or other non-literal types prevent constexpr, use const instead:

cpp

const Instruction INST_End { ... };

Even Better: Avoid Storing the Pointer at Runtime

Since you mentioned you need to iterate and find the right function — if every Instruction instance has a fixed, known-at-compile-time function, you don't need to store the pointer in the struct at all. Use a lookup table or template dispatch:

Option 1: Enum + Switch (Zero Per-Instance Overhead)

cpp

enum class OpCode { End, Add, Sub, /* ... */ };

struct Instruction {
    OpCode opcode;
    int arg_count;
    // No function pointer here!
};

void execute(const Instruction& inst, /* ... */) {
    switch (inst.opcode) {
        case OpCode::End:  INST_End_exec(inst, ...); break;
        case OpCode::Add:  INST_Add_exec(inst, ...); break;
        // ...
    }
}

This costs zero bytes per instruction instance — the dispatch is entirely compile-time resolved.

Option 2: Static Const Array (One Pointer Total, Not Per-Instance)

cpp

struct Instruction {
    int opcode;
    int arg_count;
    // No exec pointer here
};

// One global lookup table, not one pointer per instruction
const std::array<void(*)(const Instruction&, const InstToken&, 
                         ScopeState&, const std::vector<std::string>&, 
                         const std::string&), NUM_OPCODES> EXEC_TABLE = {
    INST_End_exec,
    INST_Add_exec,
    // ...
};

// Usage:
EXEC_TABLE[inst.opcode](inst, token, state, args, symbol);

Option 3: If You MUST Keep the Struct Layout

Use static constexpr inside the struct to avoid per-instance storage:

cpp

struct Instruction {
    int opcode;
    int arg_count;
    
    // Static: shared across all instances, stored once in .rodata
    static constexpr auto exec = INST_End_exec;
};

// Or use a static method that returns the right function:
static constexpr auto get_executor(OpCode op) { 
    /* return from lookup table */ 
}

Quick Checklist

ตาราง

Approach	Binary Cost Per Module	Notes
Mutable global struct with fn pointer	~0.5KB + relocation	Your current problem
`const` / `constexpr` global struct	~8 bytes in `.rodata`	Good fix
Enum + switch dispatch	0 bytes	Best for fixed mappings
Static lookup table	~8 bytes * total opcodes	Good for dynamic iteration
`static constexpr` member	0 bytes per instance	Cleanest OOP-like approach

Bottom Line

The 0.5KB isn't the pointer — it's the relocation/metadata overhead of a mutable global with a function pointer. Mark it constexpr or const, or better yet, remove the pointer from the struct entirely and use a switch or lookup table. With "a ton of little modules," that switch-based approach will save you significant space and likely run faster too (direct calls are inlineable; function pointer calls are not).

Function Pointer Struct Size Bloat