Question

How do you use a file to initialize a global const object that is too large to be created in the stack? This is my attempt so far:

// test.h
#pragma once
#include <boost/array.hpp>

typedef boost::array<int,100000> bigLut_t;
extern const bigLut_t constLut;

---

// test.cpp
#include <fstream>
#include <boost/filesystem.hpp>
#include "test.h"

bigLut_t& initializeConstLut()
{
    if( boost::filesystem::exists("my_binary_file") == false ) {
        std::ofstream outStream( "my_binary_file", ios::out | ios::binary );
        bigLut_t* tempLut = new bigLut_t;
        for(int i = 0; i < 100000; ++i) {
            // Imagine this taking a long time,
            // which is why we're using a file in the first place
            tempLut->at(i) = i;
        }
        outStream.write( reinterpret_cast<char*>(tempLut), sizeof(bigLut_t) );
        outStream.close();
        delete tempLut;
    }
    // We can't write "bigLut_t lut;" because that would cause a stack overflow
    bigLut_t* lut = new bigLut_t; // lut gets never deallocated
    std::ifstream inStream( "my_binary_file", ios::in | ios::binary );
    inStream.read( reinterpret_cast<char*>(lut), sizeof(bigLut_t) );
    inStream.close();
    return *lut;
}

const bigLut_t constLut = initializeConstLut();

AFAIK this works in a sense that constLut gets corretly initialized, but there’s a memory leak since bigLut_t* lut gets never deallocated. I tried using a smart pointer for it, but that resulted in the values in constLut being quite random. I’m baffled by the lack of information I found by trying to google the solution.

Answer 1

How did you use shared_ptr ? Try the following :

// test.h
#pragma once
#include <boost/array.hpp>

typedef boost::array<int,100000> bigLut_t;
extern const bigLut_t constLut;

---

// test.cpp
#include <fstream>
#include <boost/filesystem.hpp>
#include "test.h"

boost::shared_ptr<bigLut_t> initializeConstLut()
{
    if( boost::filesystem::exists("my_binary_file") == false ) {
        std::ofstream outStream( "my_binary_file", ios::out | ios::binary );
        bigLut_t* tempLut = new bigLut_t;
        for(int i = 0; i < 100000; ++i) {
            // Imagine this taking a long time,
            // which is why we're using a file in the first place
            tempLut->at(i) = i;
        }
        outStream.write( reinterpret_cast<char*>(tempLut), sizeof(bigLut_t) );
        outStream.close();
        delete tempLut;
    }
    // We can't write "bigLut_t lut;" because that would cause a stack overflow
    boost::shared_ptr<bigLut_t> lut(new bigLut_t); // lut gets never deallocated
    std::ifstream inStream( "my_binary_file", ios::in | ios::binary );
    inStream.read( reinterpret_cast<char*>(lut), sizeof(bigLut_t) );
    inStream.close();
    return lut;
}

const bigLut_t constLut = *(initializeConstLut().get());