Function to convert double/int into string

Question

I am a beginner in c++, so the answer maybe obvious.

I have two functions - one is able to convert int into string and the other is able convert double into string. It would be more elegant however to unite these two into one. I have tried it with the code below, but it yields an error: ‘number’ was not declared in this scope

string number_into_string (void* data, int psize)
{
    if (psize == sizeof(int))
    {
        int* number; number =(int*)data;
    }
    if (psize == sizeof(double))
    {
        double* number; number = (double*)data;
    }
    ostringstream convert;
    convert << number;
    string str = convert.str();
    return str;
}

int main()
{
    double x =1000;
    int y = 5;
    string str_x = number_into_string(&x, sizeof(x));
    string str_y = number_into_string(&y, sizeof(y));
}

Possibly related: http://stackoverflow.com/questions/10516196/append-an-int-to-a-stdstring/10516313#10516313 — hmjd, Jul 31 '13 at 14:05

score 9 · Accepted Answer · answered Jul 31 '13 at 14:06

9

Since number is declared inside the if ... { brackets, it only exists in that space. Further, you can't have one variable have two different types in C or C++.

(If you have a C++11 compiler, you don't need to do this at all, just use std::to_string which exists for most standard types).

The solution is to not use pointers, and to use a template function:

template<typename T> string number_into_string(T number)
{
    ostringstream convert;
    convert << number; 
    string str = convert.str();
    return str;
}

answered Jul 31 '13 at 14:06

Mats Petersson

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I assume you meant `ostringstream`, not `ostringstring` – Drew McGowen Jul 31 '13 at 14:07
Maybe you can add a specialized version for pointers, which output `*number`. – johnchen902 Jul 31 '13 at 14:07
@DrewMcGowen: Thanks. Not the first time I've made that mistake. – Mats Petersson Jul 31 '13 at 14:07
@johnchen902: Seems quite unnecessary. Most likely, the original number isn't a pointer in the first place - as in the example, it's got the address taken of it. – Mats Petersson Jul 31 '13 at 14:09
@derek: Yes, I'm assuming that OP has `using namespace std;` somewhere before the code, and didn't want to rewrite the code to look too different. – Mats Petersson Jul 31 '13 at 14:14
@MatsPetersson I would make the same assumption due to the use of `string`. – Derek Jul 31 '13 at 14:16

score 3 · Answer 2 · answered Jul 31 '13 at 14:05

3

Simple, use the std::to_string function.

answered Jul 31 '13 at 14:05

Some programmer dude

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@Derek if you have an irregular compiler using `std::fiber` will help. – Captain Obvlious Jul 31 '13 at 14:17

Blastfurnace · Answer 3 · 2013-07-31T14:16:04.787

0

A pair of curly braces {} create a nested scope and objects created inside that scope are not visible in the outer scope. You could rearrange your code to something like this:

string number_into_string (void* data, int psize)
{
    ostringstream convert;
    if (psize == sizeof(int))
    {
        convert << *((int*) data);
    }
    if (psize == sizeof(double))
    {
        convert << *((double*) data);
    }
    return convert.str();
}

Personally, I would use a standard library function instead of (badly) reinventing the wheel.

edited Jul 31 '13 at 14:16

answered Jul 31 '13 at 14:09

Blastfurnace

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Reinventing the wheel here is particularly messy when you consider that the standard makes no promises that `sizeof(int) != sizeof(double)`. In fact, on a system with 64-bit `int`s, they quite likely *will* be the same size. At which point things get interesting... – cHao Jul 31 '13 at 14:15

score 0 · Answer 4 · answered Jul 31 '13 at 14:11

Apart from being quite messy, the main reason for error is variable scope. Every variable has its scope of visibility, and it is not visible beyond that scope.

In function number_into_string you have 3 scopes:

function scope - this is the whole function number_into_string
first if block
second if block

Variables from function scope are visible in both if blocks, but not the other way around.

That means, in your code:

convert << number;

you're using variable which is not known (visible) in current scope.

The other thing is that you just CAN'T do this "generic values thing" in C++ without using templates or putting two copies of that "conversion code" into each if block, which woud invalidate the point of having single "conversion function".

Also, you CAN'T distinguish reliably between double and int by using sizeof.

You could pass `typeid(x)` by reference...which would give you a way to distinguish between `int` and `double`. But it still seems a rather iffy idea, unless you absolutely have to have one function for whatever reason. — cHao, Jul 31 '13 at 14:40

Function to convert double/int into string

4 Answers4