A C++ program that transpiles brainfuck code into usable C code.

bftranspiler.cpp

#include <fstream>
#include <iostream>
#include <string>
#include <map>
#include <regex>

const std::map<char, std::string> bfinstructions{
	{'>', "++ptr;"},
	{'<', "--ptr;"},
	{'+', "++*ptr;"},
	{'-', "--*ptr;"},
	{'.', "putchar(*ptr);"},
	{',', "*ptr=getchar();"},
	{'[', "while(*ptr){"},
	{']', "}"}
};

const std::regex nonbfchars("[^><+-.,[\\]]");

int main(int argc, char** argv) {
	if (argc != 3) {
		std::cerr << "Error: you must specify a file to read and a file to output.\n";
		return 1;
	}

	std::string bfprogram;

	{ // auto close the file
		std::ifstream infile(argv[1]);
		if (!infile) {
			std::cerr << "Error opening file for reading.\n";
			return 1;
		}

		std::string line;
		while (std::getline(infile, line)) {
			bfprogram += line;
		}
	}

	bfprogram = std::regex_replace(bfprogram, nonbfchars, "");

	{ // auto close the file
		std::ofstream outfile(argv[2]);
		if (!outfile) {
			std::cerr << "Error opening file for writing.\n";
			return 1;
		}

		outfile << "#include <stdio.h>\n#include <stdlib.h>\nchar arr[65535]={0};char* ptr=arr;int main(int argc, char **argv) {";

		for (std::size_t i = 0; i < bfprogram.length(); i++) {
			outfile << bfinstructions.at(bfprogram[i]);
		}

		outfile << "return 0;}";
	}

	std::cout << "Converted " << argv[1] << " to C and saved as " << argv[2] << ".\n";

	return 0;
}

helloworld.bf

taken from wikipedia
[ This program prints "Hello World!" and a newline to the screen, its
  length is 106 active command characters. [It is not the shortest.]

  This loop is an "initial comment loop", a simple way of adding a comment
  to a BF program such that you don't have to worry about any command
  characters. Any ".", ",", "+", "-", "<" and ">" characters are simply
  ignored, the "[" and "]" characters just have to be balanced. This
  loop and the commands it contains are ignored because the current cell
  defaults to a value of 0; the 0 value causes this loop to be skipped.
]
++++++++               Set Cell #0 to 8
[
    >++++               Add 4 to Cell #1; this will always set Cell #1 to 4
    [                   as the cell will be cleared by the loop
        >++             Add 2 to Cell #2
        >+++            Add 3 to Cell #3
        >+++            Add 3 to Cell #4
        >+              Add 1 to Cell #5
        <<<<-           Decrement the loop counter in Cell #1
    ]                   Loop till Cell #1 is zero; number of iterations is 4
    >+                  Add 1 to Cell #2
    >+                  Add 1 to Cell #3
    >-                  Subtract 1 from Cell #4
    >>+                 Add 1 to Cell #6
    [<]                 Move back to the first zero cell you find; this will
                        be Cell #1 which was cleared by the previous loop
    <-                  Decrement the loop Counter in Cell #0
]                       Loop till Cell #0 is zero; number of iterations is 8

The result of this is:
Cell No :   0   1   2   3   4   5   6
Contents:   0   0  72 104  88  32   8
Pointer :   ^

>>.                     Cell #2 has value 72 which is 'H'
>---.                   Subtract 3 from Cell #3 to get 101 which is 'e'
+++++++..+++.           Likewise for 'llo' from Cell #3
>>.                     Cell #5 is 32 for the space
<-.                     Subtract 1 from Cell #4 for 87 to give a 'W'
<.                      Cell #3 was set to 'o' from the end of 'Hello'
+++.------.--------.    Cell #3 for 'rl' and 'd'
>>+.                    Add 1 to Cell #5 gives us an exclamation point
>++.                    And finally a newline from Cell #6

helloworldbf.c

/*
This file (aside from the comment) is exactly as it's printed from the program. 
For proof that it's runnable, check here: https://ideone.com/8Td8Q9
*/
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
char arr[65536]={0};char* ptr=arr;int main(int argc, char **argv) {while(*ptr){*ptr=getchar();putchar(*ptr);while(*ptr){putchar(*ptr);}*ptr=getchar();putchar(*ptr);putchar(*ptr);*ptr=getchar();*ptr=getchar();*ptr=getchar();++*ptr;*ptr=getchar();--*ptr;*ptr=getchar();--ptr;++ptr;*ptr=getchar();while(*ptr){}putchar(*ptr);putchar(*ptr);}++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;while(*ptr){++ptr;++*ptr;++*ptr;++*ptr;++*ptr;while(*ptr){++ptr;++*ptr;++*ptr;++ptr;++*ptr;++*ptr;++*ptr;++ptr;++*ptr;++*ptr;++*ptr;++ptr;++*ptr;--ptr;--ptr;--ptr;--ptr;--*ptr;}++ptr;++*ptr;++ptr;++*ptr;++ptr;--*ptr;++ptr;++ptr;++*ptr;while(*ptr){--ptr;}--ptr;--*ptr;}++ptr;++ptr;putchar(*ptr);++ptr;--*ptr;--*ptr;--*ptr;putchar(*ptr);++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;++*ptr;putchar(*ptr);putchar(*ptr);++*ptr;++*ptr;++*ptr;putchar(*ptr);++ptr;++ptr;putchar(*ptr);--ptr;--*ptr;putchar(*ptr);--ptr;putchar(*ptr);++*ptr;++*ptr;++*ptr;putchar(*ptr);--*ptr;--*ptr;--*ptr;--*ptr;--*ptr;--*ptr;putchar(*ptr);--*ptr;--*ptr;--*ptr;--*ptr;--*ptr;--*ptr;--*ptr;--*ptr;putchar(*ptr);++ptr;++ptr;++*ptr;putchar(*ptr);++ptr;++*ptr;++*ptr;putchar(*ptr);return 0;}