cpp_smartpointers.rst

Smart Pointers

Table of Contents

• Custom Deleters
• std::make_shared and std::make_unique

Custom Deleters

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <iostream>
#include <string>
#include <exception>
#include <memory>

using FilePtr = std::unique_ptr<FILE, int (*)(FILE *)>;

constexpr void assert_that(bool statement, const char *msg) {
    if (!statement) {
        throw std::runtime_error(msg);
    }
}

int main(int argc, char *argv[]) {

    assert_that(argc == 2, "Usage: command [path]");

    FILE *f = nullptr;
    f = fopen(argv[1], "r+");
    assert_that(f, strerror(errno));

    // assign FILE* to a unique_ptr
    FilePtr fptr{f, fclose};
    assert_that(!!fptr, strerror(errno));
    assert_that(fseek(fptr.get(), 0, SEEK_END) == 0, strerror(errno));

    long size = ftell(fptr.get());
    assert_that(size >=0, strerror(errno));
    rewind(fptr.get());

    // using unique_ptr to create a buffer instead of using malloc
    std::unique_ptr<char[]> buf{ new char[size + 1]{0} };
    assert_that(!!buf, strerror(errno));

    size_t r = fread(buf.get(), 1, size, fptr.get());
    assert_that(r == size, "Reading error");
    std::cout << buf.get();
end:
    return 0;
}

std::make_shared and std::make_unique

std::make_shared and std::make_unique are the recommended ways to create smart pointers because compilers do guarantee the order of executions, which may introduce memory leaks when an exception is thrown. For example, the compilers may call new T, then raise(), and so on before foo is called. In this case, std::unique_ptr does not know the pointer T yet, so it is still on the heap.

using uptr = std::unique_ptr<T>;

bool raise() {
    throw std::exception();
    return true;
}

foo(uptr(new T), raise(), uptr(new T));