to get value from String variable containing a decimal number representing a value of about 64 gigabyte

Question

I have a question in conversion of String into value.

Following is the sample code I tried.

I am trying this on C++ Builder XE4.

String strSize = L"64420392960"; // 64GB

size_t size;

size = strSize.ToDouble(); // returns 4290850816

char *end_ptr;
size = strtol(AnsiString(strSize).c_str(), &end_ptr, 10); // returns 0

Both of ToDouble() and strtol() didn't work.

I understand that strtol didn't work because long type is up to 4.3GB.

Are there any function in C++ Builder XE4, with which I can convert the strSize into size_t value when I treat 64GB or several hundreds of GB (eg 500GB)?

Answer 1

You don't have a 64GB string, which would be a string that's 64 gigabytes in size. You have a string containing a decimal number representing a value of about 64 gig.

The standard function you're looking for is strtoll , which returns a long long result. It's been a standard C function since the 1999 standard, and if I'm not mistaken was added to the C++ standard as of 2011.

(The question is whether C++ Builder XE4 supports strtoll .)

If size_t on your system is only 32 bits, there's no way to get a size_t value that big. If it's 64 bits, strtoll should do the trick.

Answer 2

The problem is that the VCL String object (which is apparently an alias for the VCL AnsiString UnicodeString class) returns a double with the value 64420392960.0 which gets converted into an integral type but the size_t is a 32-bit unsigned type so it keeps only the lower 32 bits of the value.

Instead of a type size_t try an unsigned long long or plain old long long if your toolchain supports it.

Answer 3

Other people have explained why your original code does not work. There is another alternative: use __int64 instead of long long . The VCL has support for __int64 , eg:

String strSize = L"64420392960";
__int64 size = StrToInt64(strSize);

Don't use double unless you really need floating-point precision.