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[flexbuffers] Add "AlignedBlob", a version of "Blob" with explicit alignment. #8993

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Merged
dbaileychess merged 1 commit into from
Mar 23, 2026
Merged

[flexbuffers] Add "AlignedBlob", a version of "Blob" with explicit alignment. #8993

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20 changes: 17 additions & 3 deletions include/flatbuffers/flexbuffers.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1207,11 +1207,20 @@ class Builder FLATBUFFERS_FINAL_CLASS {
String(str);
}

size_t AlignedBlob(const void* data, size_t len, BitWidth alignment) {
// The requested alignment must not be smaller than the one required to
// store the length.
return CreateAlignedBlob(data, len, 0, FBT_BLOB,
std::max(alignment, WidthU(len)));
}
size_t AlignedBlob(const std::vector<uint8_t>& v, BitWidth alignment) {
return AlignedBlob(v.data(), v.size(), alignment);
}
size_t Blob(const void* data, size_t len) {
return CreateBlob(data, len, 0, FBT_BLOB);
}
size_t Blob(const std::vector<uint8_t>& v) {
return CreateBlob(v.data(), v.size(), 0, FBT_BLOB);
return Blob(v.data(), v.size());
}

void Blob(const char* key, const void* data, size_t len) {
Expand Down Expand Up @@ -1693,11 +1702,16 @@ class Builder FLATBUFFERS_FINAL_CLASS {

size_t CreateBlob(const void* data, size_t len, size_t trailing, Type type) {
auto bit_width = WidthU(len);
auto byte_width = Align(bit_width);
return CreateAlignedBlob(data, len, trailing, type, bit_width);
}

size_t CreateAlignedBlob(const void* data, size_t len, size_t trailing,
Type type, BitWidth alignment) {
auto byte_width = Align(alignment);
Write<uint64_t>(len, byte_width);
auto sloc = buf_.size();
WriteBytes(data, len + trailing);
stack_.push_back(Value(static_cast<uint64_t>(sloc), type, bit_width));
stack_.push_back(Value(static_cast<uint64_t>(sloc), type, alignment));
return sloc;
}

Expand Down
33 changes: 25 additions & 8 deletions tests/flexbuffers_test.cpp
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
#include "flexbuffers_test.h"

#include <memory>
#include <limits>

#include "flatbuffers/flexbuffers.h"
Expand All @@ -13,6 +14,13 @@ namespace tests {
// Shortcuts for the infinity.
static const auto infinity_d = std::numeric_limits<double>::infinity();

static bool IsAligned(const void* ptr, std::size_t alignment) {
void* p = const_cast<void*>(ptr);
std::size_t space = 2 * alignment;
void* q = std::align(alignment, alignment, p, space);
return q != nullptr && p == ptr && space == 2 * alignment;
}

void FlexBuffersTest() {
flexbuffers::Builder slb(512,
flexbuffers::BUILDER_FLAG_SHARE_KEYS_AND_STRINGS);
Expand All @@ -29,7 +37,10 @@ void FlexBuffersTest() {
slb.IndirectFloat(4.0f);
auto i_f = slb.LastValue();
uint8_t blob[] = {77};
slb.Blob(blob, 1);
uint32_t aligned_blob[] = {88, 99};
slb.Blob(blob, sizeof blob);
slb.AlignedBlob(aligned_blob, sizeof aligned_blob,
flexbuffers::BIT_WIDTH_32);
slb += false;
slb.ReuseValue(i_f);
});
Expand Down Expand Up @@ -62,7 +73,7 @@ void FlexBuffersTest() {
auto map = flexbuffers::GetRoot(slb.GetBuffer()).AsMap();
TEST_EQ(map.size(), 7);
auto vec = map["vec"].AsVector();
TEST_EQ(vec.size(), 6);
TEST_EQ(vec.size(), 7);
TEST_EQ(vec[0].AsInt64(), -100);
TEST_EQ_STR(vec[1].AsString().c_str(), "Fred");
TEST_EQ(vec[1].AsInt64(), 0); // Number parsing failed.
Expand All @@ -80,9 +91,15 @@ void FlexBuffersTest() {
auto blob = vec[3].AsBlob();
TEST_EQ(blob.size(), 1);
TEST_EQ(blob.data()[0], 77);
TEST_EQ(vec[4].IsBool(), true); // Check if type is a bool
TEST_EQ(vec[4].AsBool(), false); // Check if value is false
TEST_EQ(vec[5].AsDouble(), 4.0); // This is shared with vec[2] !
TEST_EQ(vec[4].IsBlob(), true);
auto aligned_blob = vec[4].AsBlob();
TEST_EQ(aligned_blob.size(), 8);
TEST_EQ(reinterpret_cast<const uint32_t*>(aligned_blob.data())[0], 88);
TEST_EQ(reinterpret_cast<const uint32_t*>(aligned_blob.data())[1], 99);
TEST_EQ(IsAligned(aligned_blob.data(), 4), true);
TEST_EQ(vec[5].IsBool(), true); // Check if type is a bool
TEST_EQ(vec[5].AsBool(), false); // Check if value is false
TEST_EQ(vec[6].AsDouble(), 4.0); // This is shared with vec[2] !
auto tvec = map["bar"].AsTypedVector();
TEST_EQ(tvec.size(), 3);
TEST_EQ(tvec[2].AsInt8(), 3);
Expand All @@ -107,9 +124,9 @@ void FlexBuffersTest() {
TEST_EQ(vec[2].MutateFloat(2.0f), true);
TEST_EQ(vec[2].AsFloat(), 2.0f);
TEST_EQ(vec[2].MutateFloat(3.14159), false); // Double does not fit in float.
TEST_EQ(vec[4].AsBool(), false); // Is false before change
TEST_EQ(vec[4].MutateBool(true), true); // Can change a bool
TEST_EQ(vec[4].AsBool(), true); // Changed bool is now true
TEST_EQ(vec[5].AsBool(), false); // Is false before change
TEST_EQ(vec[5].MutateBool(true), true); // Can change a bool
TEST_EQ(vec[5].AsBool(), true); // Changed bool is now true

// Parse from JSON:
flatbuffers::Parser parser;
Expand Down
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