C++ String.Format implementation error

Question

I am trying to implement the same function as C#'s String.Format in C++. However, I have a problem with the output in some cases:

#include <iostream>
#include <string>

class format_base
{
public:
    format_base(const char *base) : argp_(0)
    {
        base_ = (char*)calloc(4097, sizeof(char));
        sz_ = 4097;
        memcpy(base_, base, strlen(base));
    };
    ~format_base()
    {
        free(base_);
    }
    format_base &arg(const char *argument)
    {
        if ((strlen(base_) - 3) + strlen(argument) > sz_)
        {
            base_ = (char*)realloc(base_, (sz_ *= 2));
        }
        std::string elim = "{";
        elim.append(std::to_string(argp_++));
        elim.append("}");
        std::string tbase = base_;
        size_t f = tbase.find(elim.c_str());
        if (f != std::string::npos)
        {
            tbase.replace(f, elim.size(), argument);
        }
        memcpy(base_, tbase.c_str(), tbase.length());
        return *this;
    }
    char *value()
    {
        return base_;
    }
    char *operator()()
    {
        return base_;
    }
private:
    char *base_;
    size_t argp_, sz_;
};

format_base &format(const char *base)
{
    return *new format_base(base);
}

int main()
{
    std::cout << format("Hello {0}").arg("a")(); // Prints "Hello a0} "
    std::cout << format(" Hello {0}").arg("ab")(); // Prints " Hello ab} "
    std::cout << format(" Hello {0}\n").arg("abc")(); // Prints " Hello abc\n"
    std::cout << format("Hello {0}\n").arg("a")(); // Prints "Hello a\n}"
    std::cout << format("Hello {0}\n").arg("ab")(); // Prints "Hello ab\n"
    std::cout << format("Hello {0}\n").arg("abc")(); // Prints "Hello abc\n"
    getchar();
}

Total output:

Hello a0} Hello ab} Hello abc

Hello a

}

Hello ab

_

Hello abc

_

I am very confused and I would be very grateful if you could help

There's already a std::to_string function that returns a proper std::string. No need to make another that returns a C string. — Qaz
– Qaz, Commented Feb 12, 2014 at 17:35
Are you sure you don't want to use std::string everywhere instead of manually managing memory? — Sebastian Redl
– Sebastian Redl, Commented Feb 12, 2014 at 17:35

Peter · Accepted Answer · 2014-02-12 17:44:01Z

1

I guess you forgot to set the string length properly. The "bad" output is just the result string and the last characters of the old string. You need to cut off the string if it is shortened by the replacement.

memcpy(base_, tbase.c_str(), tbase.length());

length() doesn't count the terminating \0, so you probably want

strcpy(base_, tbase.c_str());

or

memcpy(base_, tbase.c_str(), tbase.length()+1);

answered Feb 12, 2014 at 17:44

Peter

5,7481 gold badge26 silver badges44 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

marcinj · Accepted Answer · 2014-02-12 17:44:44Z

1

You should replace:

    memcpy(base_, tbase.c_str(), tbase.length());

with:

    strcpy(base_, tbase.c_str());

where have you planned to free memory from this allocations? :

return *new format_base(base);
char *s = (char*)calloc(2, sizeof(char));

answered Feb 12, 2014 at 17:44

marcinj

50.3k10 gold badges85 silver badges103 bronze badges

Collectives™ on Stack Overflow

C++ String.Format implementation error

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related