Fix incorrect byte size calculation in NimBLEAttValue::setValue for container types

[srgg]

Correct byte size handling for container types in NimBLEAttValue::setValue

Summary

Improves NimBLEAttValue::setValue so that all container types correctly calculate byte size before updating an ATT value. The fix fully addresses incorrect buffer sizing when the container element type is larger than one byte.

Problem

The previous implementation used v.size() as the byte length. This works only when elements are 1 byte. For multi-byte element types (for example uint16_t), this caused:

• Incorrect memory allocation
• Data truncation or potential buffer overflow
• Wrong BLE payload size reported to the stack

Example:
std::vector<uint16_t> with 10 elements

• Before: 10 bytes (incorrect)
• Now: 20 bytes (correct: 10 × 2 bytes)

Changes Made

1. Added Has_value_type trait (lines 76–82)

   • C++11-compatible SFINAE trait that detects presence of value_type
   • Works with all standard containers (std::vector, std::array, std::string, etc.)

2. Updated pre-C++17 templates (lines 282–314)

   • Split into two overloads:
     • With value_type: size in bytes = size() × sizeof(value_type)
     • Without value_type: use size() directly (assumed to already represent bytes)

3. Updated C++17+ template (lines 324–329)

   • Uses nested if constexpr to branch at compile time
   • Same logic as pre-C++17 implementation but cleaner and safer

Behavior Examples

Type	size()	Byte calculation	Result
std::vector<uint8_t>	10	10×1	10 ✓
std::vector<uint16_t>	10	10×2	20 ✓
std::vector<uint32_t>	10	10×4	40 ✓
std::string	chars	Has_c_str_length override	✓
Custom container without value_type	N elements	Uses size() directly	✓

Impact

• Correct, safe sizing for all container types
• Avoids data corruption and memory errors
• No API changes required
• Backwards compatible behavior for byte-sized types

[h2zero]

Thanks! I hadn't thought of this use case before. Could you also apply this to the NimBLERemoteAttribute::writeValue and NimBLECharacteristic::notify/NimBLECharateristic::indicate templates as well?

[srgg]

e80a7fc extends the previous fix for NimBLEAttValue::setValue to additional template methods across the NimBLE library. These changes ensure that containers with multi-byte element types are properly sized when written, notified, or indicated.

[srgg]

Hi @h2zero, have you had a chance to review the recent changes? Are they good to go, or do you see anything that needs adjustment?

[h2zero]

Sorry, been busy. Changes look good, I will merge soon.