VarargsHelper.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreTypes.h"
#include "Misc/CString.h"
#include "HAL/UnrealMemory.h"
 
#include <cstdarg>
 
template <typename SerializeFuncType>
bool GrowableLogfV(const TCHAR* Fmt, va_list Args, const SerializeFuncType& SerializeFunc)
{
    // Allocate some stack space to use on the first pass. This should be sufficient for most strings.
    constexpr int DefaultBufferSize = 512;
 
    TCHAR   StackBuffer[DefaultBufferSize];
    TCHAR*  Buffer = StackBuffer;
    int32   BufferSize = DefaultBufferSize;
    TCHAR*  AllocatedBuffer = nullptr;
    int32   Length = -1;
 
    for (;;)
    {
        va_list ArgsCopy;
        va_copy(ArgsCopy, Args);
        Length = FCString::GetVarArgs(Buffer, BufferSize, Fmt, ArgsCopy);
        va_end(ArgsCopy);
 
        if (Length >= 0 && Length < BufferSize - 1)
        {
            break;
        }
 
        // Make increasingly large allocations on the heap until we can hold the formatted string.
        // We need to use SystemMalloc here, as GMalloc might not be safe.
        FMemory::SystemFree(AllocatedBuffer);
        BufferSize *= 2;
        Buffer = AllocatedBuffer = (TCHAR*) FMemory::SystemMalloc(BufferSize * sizeof(TCHAR));
        if (Buffer == nullptr)
        {
            return false;
        }
    }
    Buffer[Length] = TEXT('\0');
 
    SerializeFunc(Buffer);
    FMemory::SystemFree(AllocatedBuffer);
    return true;
}
 
// This macro expects to be used in a variadic function where the parameter list ends with
// a `Fmt` text string, followed by the `...` variadic indicator. The passed-in code snippet
// will have access to the resulting string via a variable named `Buffer`.
#define GROWABLE_LOGF(SerializeSnippet) \
    { \
        va_list Args; \
        va_start(Args, Fmt); \
        GrowableLogfV(Fmt, Args, [&](TCHAR* Buffer) { SerializeSnippet; }); \
        va_end(Args); \
    }