[gdal-dev] constexpr - was Re: [gdal-commits] r41044 - trunk/gdal/port

[Andrew C Aitchison]

On Fri, 15 Dec 2017, Even Rouault wrote:

> Kurt,
>
> It doesn't look like constexpr is available in VS2013 (at least not in all sub-versions of it):
> https://stackoverflow.com/questions/20264644/constexpr-not-compiling-in-vc2013
>
> Personnally, I don't see it as a super needed feature that would justify dropping VS2013
> support. Especially in the below use cases where it doesn't bring anything more than const,
> except more characters to type/read. I'd bet any sane compiler in -O2 will compile the source
> to the same machine code in all those cases

As a C programmer, writing gdal device drivers is teaching me much about 
C++, but I'm finding it difficult to understand why I should need to care
whether a const is a constexpr.

Except in the rare case where I might want a value that cannot be initialised
at compile time, but my code must not change once it has been initialised,
I don't see the need to distinguish between the two and wish that a const
was a constexpr unless otherwise declared.

I worry that Kurt's improvements are turning C++ into a foreign language 
for me.

> On vendredi 15 décembre 2017 07:04:23 CET you wrote:
>> Author: goatbar
>> Date: 2017-12-15 07:04:23 -0800 (Fri, 15 Dec 2017)
>> New Revision: 41044
>>
>> Modified:
>>    trunk/gdal/port/cpl_conv.cpp
>> Log:
>> const -> constexpr for things known at compile time in cpl_conv.cpp
>>
>>
>> Modified: trunk/gdal/port/cpl_conv.cpp
>> ===================================================================
>> --- trunk/gdal/port/cpl_conv.cpp	2017-12-15 14:37:43 UTC (rev 41043)
>> +++ trunk/gdal/port/cpl_conv.cpp	2017-12-15 15:04:23 UTC (rev 41044)
>> @@ -340,8 +340,8 @@
>>  /************************************************************************/
>>
>>  // ASCII characters.
>> -static const char knLF = 10;
>> -static const char knCR = 13;
>> +constexpr char knLF = 10;
>> +constexpr char knCR = 13;
>>
>>  /**
>>   * Reads in at most one less than nBufferSize characters from the fp
>> @@ -666,7 +666,7 @@
>>  /*      the line.                                                       */
>>  /* -------------------------------------------------------------------- */
>>      char *pszRLBuffer = nullptr;
>> -    const size_t nChunkSize = 40;
>> +    constexpr size_t nChunkSize = 40;
>>      char szChunk[nChunkSize] = {};
>>      size_t nChunkBytesRead = 0;
>>      int nBufLength = 0;
>> @@ -1984,8 +1984,8 @@
>>  /*                            CPLDMSToDec()                             */
>>  /************************************************************************/
>>
>> -static const char *sym = "NnEeSsWw";
>> -static const double vm[] = { 1.0, 0.0166666666667, 0.00027777778 };
>> +constexpr char sym[] = "NnEeSsWw";
>> +constexpr double vm[] = { 1.0, 0.0166666666667, 0.00027777778 };
>>
>>  /** CPLDMSToDec */
>>  double CPLDMSToDec( const char *is )
>> @@ -2605,7 +2605,7 @@
>>  /* -------------------------------------------------------------------- */
>>  /*      Prepare buffer.                                                 */
>>  /* -------------------------------------------------------------------- */
>> -    const size_t nBufferSize = 1024 * 1024;
>> +    constexpr size_t nBufferSize = 1024 * 1024;
>>      GByte *pabyBuffer = static_cast<GByte
>> *>(VSI_MALLOC_VERBOSE(nBufferSize)); if( pabyBuffer == nullptr )
>>      {
>>
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>
>
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