CODEWITHSUNDEEP
c++c++11
Tharindu Kumara
Tharindu Kumara

Reputation: 4458

Set the digits in a float after decimal point

I want to write a c++ function which converts a float to const char *. Here in this function, an argument is passed to specify the decimal places that output value should represent. I have come up this functions and it works nicely.

I would like to whether, there are any better ways write this function?.

static const char* getString(float value, int decimalPlaces)
{
     char strValue[sizeof value];
     sprintf(strValue, "%.%df", value, decimalPlaces);
     return strValue;
}

Upvotes: 0

Views: 821

Answers (3)

Some programmer dude
Some programmer dude

Reputation: 409472

There are two major problems with your function.

To begin with the size of float is on most platform four bytes, which means your array is only four elements. Not enough for all the digits of your floating point value. Writing out of bounds will lead to undefined behavior.

The second problem is that you return a pointer to a local variable. The array strValue will go out of scope once the function returns, and the pointer will now be a so-called stray pointer. Dereferencing it will also lead to undefined behavior.

In short, your function does not "work nicely". It doesn't really work at all.

The obvious solution is to use std::string and std::to_string to convert the floating point value to a string. Then when you need a C-style null-terminated string (for whatever reason) you use the strings c_str() member function to get such a pointer.

If you need a specific number of decimals, use an std::ostringstream and standard I/O manipulators to format the string the way you want. If your goal is to get a string to output you can of course skip this step, and use the manipulators directly when writing the output.

Upvotes: 6

Starl1ght
Starl1ght

Reputation: 4493

There is a problem with your function, since char strValue[] is deleted once you leave getMString and you return pointer to deleted data, this is UB.

I suggest not to invent anything, but use C++11 std::to_string(float)

Upvotes: 2

NathanOliver
NathanOliver

Reputation: 181057

When you return strValue; you are going to return a pointer to a local variable. That means when the function ends that variable goes away and now you have a pointer to nothing. Using that pointer is undefined behavior.

You are either going to have to use new[] to allocate the buffer and then you need to remember to delete[]the pointer in the call site or use something like a std::string and let the string itself handle the memory management.

Personally I would use a std::string and a std::stringstream like:

static std::string getMString(float value, int decimalPlaces)
{
    std::stringstream stream;
    stream << std::fixed << std::setprecision(decimalPlaces) << value;
    return stream.str();
}

Upvotes: 2

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