C++编程：打造可移植嵌入式系统的关键技术与实践

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"《C++设计与构建可移植系统》一书中，作者Günter Obiltschnig，作为Applied Informatics领域的专家，深入探讨了在C++中设计和开发可移植系统的必要性和挑战。C++语言因其覆盖了从低级到高级编程的全面特性，使得它成为编写跨平台软件的理想选择。然而，在嵌入式系统工程中，代码的可移植性常常被忽视，随着软件复杂性的增加和硬件的日益标准化，这可能导致软件移植到新平台时出现问题。文章首先介绍了嵌入式系统的软件复杂性提升，以及硬件角色的转变，定制硬件设计逐渐被现成组件所取代。这种趋势强调了软件的可移植性愈发关键，因为不同的CPU和控制器之间的兼容性要求软件能够无缝适应多种硬件环境。为了克服这个问题，作者提出了在C++中实现软件可移植性的工具和技术。通过利用C++特性，如封装（encapsulation）来隔离平台依赖的部分，包括编译器/语言差异、操作系统接口以及输入/输出操作，作者展示了如何确保最终系统的跨平台兼容性。这包括： 1. **语言特性**：理解并合理利用C++的模板、抽象类、泛型编程等特性，以避免硬编码特定平台细节。 2. **库和框架**：利用跨平台的C++标准库或第三方库，如Boost库，减少平台依赖。 3. **模块化设计**：将代码分解为可重用的模块，每个模块独立于特定平台实现。 4. **条件编译**：根据编译时的宏定义或预处理器指令来调整代码，处理不同平台的差异。 5. **异常处理和错误管理**：使用一致的错误处理策略，确保在各种平台上都能正确响应问题。 6. **操作系统接口抽象**：通过操作系统接口抽象层（如POSIX API）统一对外部服务的调用方式。 7. **测试和验证**：进行广泛的测试，包括单元测试和系统集成测试，确保在不同环境中都能正常工作。通过遵循这些原则和实践，开发者可以在C++中设计出既能满足性能要求又能跨越多种硬件平台的可移植系统，从而提高开发效率并降低维护成本。"

parse some of its own standard library header files. The automatic instantiation of templates

in conjunction with shared libraries caused nightmares on many platforms. Nesting of

templates, member templates, and partial specialization can cause problems even today.

Furthermore, the implementations of the standard library that shipped with the various

compilers had bugs on their own, up to a degree that made them practically unusable for use

in real-world applications (the implementation that came with Microsoft Visual C++ 6 with

its multithreading issues comes to mind here). A good alternative in such a case is STLport,

an open-source, portable, high-quality implementation of the C++ standard library.

Life was — and even sometimes still is — quite hard for developers wanting to write both

standards-compliant and portable C++ code. Fortunately, with the latest C++ compiler

releases, things are finally getting better.

2.1.2 Implementation-Defined, Unspecified and Undefined Behavior

The C++ standard specifies the exact (required) behavior for most, but not for all language

elements. Certain language constructs are described as implementation-defined, unspecified or

undefined.

Implementation-defined means that the compiler writer is free to implement a certain

language construct in any way he sees appropriate, as long as the exact behavior is consistent,

documented, and the compilation succeeds. The standard may specify a number of allowable

behaviors from which to choose one, or it may leave it entirely up to the compiler writer.

Unspecified is similar, except that the behavior of the implementation need not be

documented and need not even be consistent.

Undefined behavior means that the standard does not place any requirements whatsoever on

the implementation. The compiler may even fail when compiling such a language construct,

or the program may silently produce incorrect results.

The rationale behind all this is that it allows the compiler writer to implement certain

constructs in a way that works best for the target platform. The downside of all this is that it

opens up opportunities for portability problems. Code that works as expected on one platform

can produce different, unexpected results on another platform. Even more so, the behavior

can be different depending on whether the compiler's optimizer has been used, or not.

An example for unspecified behavior is the order in which function arguments are evaluated

in a function call. Consider the following example:

void f(int a, int b, int c)

{

cout << "a = " << a << ", b = " << b << ", c = " << c << endl;

}

int main(int argc, char** argv)

{

int i = 0;

f(i++, i++, i++);

return 0;

}

This example will yield surprisingly different results on different platforms. For example, on

Mac OS X, using GCC 3.3, the program fragment yields the following output

a = 0, b = 1, c = 2

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C++编程：打造可移植嵌入式系统的关键技术与实践

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