[英]C++: Iterating over a string vector and using the index for putenv
I have a vector of strings which was created from parsing a config file.我有一个字符串向量,它是通过解析配置文件创建的。 All the strings should be in the format
key=value
.所有字符串都应采用
key=value
格式。 I'd like to iterate over the vector, and use the putenv function to set an environment variable to the key-value pair.我想遍历向量,并使用 putenv 函数将环境变量设置为键值对。
The code:编码:
for(auto it = settings.begin(); it != settings.end(); it++) {
try {
auto i = it - settings.begin();
cout << i << endl;
putenv(settings.at(i));
} catch (...) {
cout << "Config is not in the format key=value ... please correct" << endl;
}
}
This throws the error:这会引发错误:
cannot convert ‘__gnu_cxx::__alloc_traits<std::allocator<std::basic_string<char> > >::value_type {aka std::basic_string<char>}’ to ‘char*’ for argument ‘1’ to ‘int putenv(char*)’
I'm very new to C++, and all these variable types and pointers are confusing me.我对 C++ 很陌生,所有这些变量类型和指针都让我感到困惑。
You're mixing C and C++ stuff.你正在混合 C 和 C++ 的东西。
std::string
.std::string
。putenv
is an "old" function expecting a pointer to a char
buffer, ie a C-string. putenv
是一个“旧”函数,它需要一个指向char
缓冲区的指针,即一个 C 字符串。 Fortunately, std::string
makes it easy to get one of those :幸运的是,
std::string
可以轻松获得其中之一:
putenv(settings.at(i).c_str());
// ^^^^^^^^
However, there is still a problem there.但是,那里仍然存在问题。
putenv
takes "ownership" of the buffer you give it, and expects it to last "forever". putenv
获取您提供的缓冲区的“所有权”,并希望它“永远”持续。 Yours won't;你的不会; it'll only be there until the
std::string
is modified or destroyed.它只会在
std::string
被修改或销毁之前存在。 Not good enough!还不够好!
Conventionally, we use C's strdup
here to allocate a copy of the char
buffer.按照惯例,我们在这里使用 C 的
strdup
来分配char
缓冲区的副本。 It's then putenv
's (or the OS's) responsibility to free it later.然后
putenv
(或操作系统)有责任稍后释放它。
putenv(strdup(settings.at(i).c_str()));
// ^^^^^^^ ^
Since putenv
is not in the C or C++ standard, but is provided by POSIX, you should check the manpages for your operating system to make sure you're using it correctly.由于
putenv
不在 C 或 C++ 标准中,而是由 POSIX 提供,因此您应该检查操作系统的联机帮助页以确保正确使用它。
The error is caused by your call to putenv(), which expects a pointer to char
's.该错误是由您调用 putenv() 引起的,它需要一个指向
char
的指针。 Your vector contains C++ strings (std::string)...您的向量包含 C++ 字符串 (std::string)...
You can try this:你可以试试这个:
for (auto setting : settings) {
// ... putenv() is inconsistent across compilers, so use setenv() instead...
std::string key = item.substr( 0, item.find_first_of( "=" ) ) ;
std::string val = item.substr( key.length()+1 ) ;
if ( setenv( key.c_str(), val.c_str(), 1 ) != 0 ) {
cerr << "setenv(" << key << ',' << val << ") has failed: " << strerror( errno ) << endl;
}
}
From what I've read, putenv
should be avoided in new code, and setenv
should be used, as it makes a copy of its arguments, regardless of compiler version.从我读过,
putenv
应避免在新的代码,并setenv
应该使用,因为它使得它的参数的副本,无论编译器版本。
( setenv
is in stdlib.h) (
setenv
在 stdlib.h 中)
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