Implementations of the C++ Standard Library in... C. Can creatures armed without wings engage in flight?
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Implementation of the C++ Standard Library in C. Can creatures armed without wings engage in flight?



Functions from the <algorithm> header are currently being implemented as macros, often requiring an explicit passing of the type being used.

Example usage, which removes all vowels from a string:

#include <cstl/algorithm.h>
#include <cstl/string.h>
#include <stdio.h>
int isvowel(char c);

int main() {
    string* s = string_create_cstr("Hello world");
    char* last;
    algorithm_remove_if(char*, string_begin(s), string_end(s), isvowel, last);
    string_erase_range(s, last, string_end(s));
    printf("%s\n", string_cstr(s));

int isvowel(char c) {
    char vowels[] = {'a','e','i','o','u','A','E','I','O','U'};
    char* found;
    algorithm_find(vowels, vowels + sizeof(vowels), c, found);
    return found != (vowels + sizeof(vowels));

For more, check out algorithm.h.


Emulating the C++ <vector> in C is difficult, but macros allow us some room to play. Two macros, declare_vector_type and define_vector_type, allow us to explicitly provide a type (much like a template) and construct the rest according to our needs. These are built on declare_vector and define_vector, which allow more flexibility and power with the naming of containers and functions.

Since the declaration of the vector struct data type and associated functions should ideally belong in a header file, it is advised to use the declare_vector within one, and use define_vector in a matching .c file. For example, to create an int vector, one might write:

 * vector_int.h 
#ifndef VECTOR_INT_H
#define VECTOR_INT_H

#include <cstl/vector.h>


 * vector_int.c
#include "vector_int.h"
#include <cstl/vector.h>


This will then expand into the appropriate vector struct (e.g. vector_int) with matching functions. All functions are appended with the type, with the general format vector_[TYPE]_[FUNCTION]. Therefore, to push_back on an int vector, one would call vector_int_push_back.

As vector relies on dynamically-allocated memory via malloc, a set of functions are used to allocate and release a vector after it has finished being used. To supplant a user-defined malloc, one can define the CSTL_CUSTOM_MEMORY macro and pass it in to the compiler, and implement the following functions: cstl_malloc, cstl_realloc, and cstl_free. To understand why this works, there is essentially a kind of conditional compilation going on. These functions are defined internally within a source file only if CSTL_CUSTOM_MACRO is not defined. Therefore, defining the macro frees up the implementation, allowing the user to supplant one easily. Through gcc, this might be done as cc -DCSTL_CUSTOM_MACRO ....

To sum up, here is some example usage with the declared vector_int above:

#include "vector_int.h"

int main() {
    //Initializing a new vector
    vector_int v;

    //Inserting 5 of the first elements from an array
    int a[] = {10, 20, 30, 40, 50};
    vector_int_insert_range(&v, vector_int_begin(&v), a, a + (sizeof(a) / sizeof(a[0])));

    //Printing vector contents to the console
    for(int* it = vector_int_begin(&v); it != vector_int_end(&v); ++it) {
        printf("%d\n", *it);


Further documentation on the currently implemented functions can be found in vector.h.


The flexible, dynamically-allocated C++ std::string is emulated by expanding on the vector macro, and adding additional functions useful for interaction with C-strings. All functions available to vectors are equally available to the C-STL string type (e.g. string_push_back(char c)). Information on the additional functions can be found in string.h.

Example usage:

#include <cstl/string.h>

int main() {
    //Initializing our string to a C-string
    string s;
    string_init_cstr(&s, "Hello world");

    //Setting our string to a new C-string
    string_asn_cstr(&s, "I need a string that can expand!");

    //And concatenating it with a C-string
    string_cat_cstr(&s, " And concatenate, too!");

    //Exporting our string as a C-string
    printf("%s", string_cstr(&s));


Useful C Additions

Alongside constructs which exist in the C++ STL and standard library, some functions have been written to better interact with “native” C constructs (such as C-strings).


This module offers an interface to easily reason through heap-allocated C-strings, which may be struct members. The functions ensure that an initialized C-string is either NULL or a char* to a valid null-terminated string. In this paradigm, testing if a string is empty/non-existent is as easy as testing the pointer itself, and no unnecessary memory has to be allocated to simply represent an empty C-string. This works particularly well, as NULL is a valid argument for free().

The only minor cost is a branch in C-string assignment (cstring_asn), in order to accept a valid, intialized string (which may be NULL). The reasoning behind this is to allow implementing the assignment of a struct to another struct easily; since it is possible for the string to be NULL, we don’t want cstring_asn to break assuming the value is allocated.

Example usage:

#include <cstl/cstring.h>

int main() {
    //Initializing a C-string
    char* s;

    //Assigning it a value
    cstring_asn(s, "A new value!");

    //Printing it as we usually would
    printf("%s", s);

A small set of functions are also avaliable within this header, which are much more useful globally. safestrcpy and safestrcat allow specifying a limit to protect from overwriting buffers, where lim - 1 characters are copied max.

Example usage:

#include <cstl/cstring.h>
#define BUF_MAX 8
char buf[BUF_MAX];

int main() {
    char s[] = "I can't not easily overrun the buffer above";
    //Perform a safestrcpy to prevent it!
    safestrcpy(buf, s, BUF_MAX);

    //Let's check to see if our buffer is safe: (prints "I can't")

More can be found in cstring.h.

NOTE: This module is constantly changing and improving, this document attempts to work as a good starting point with as much as correct as possible, but the source of truth should rest in the library’s comments themselves. This README is not guaranteed to contain up-to-date information with the latest master build.


The test/ directory contains tests which intend to exercise typical usage. To run all tests, simply use the command make test, which also includes diagnostic information from valgrind to ensure resources are being freed properly. The simple test framework Unity is used to write them.


This code has been licensed under the GNU General Public License v3.0.