C++ g++;错误:‘;占位符’;不是使用命名空间std::占位符的命名空间名称;
我正在努力理解C++ g++;错误:‘;占位符’;不是使用命名空间std::占位符的命名空间名称;,c++,functional-programming,stdbind,C++,Functional Programming,Stdbind,我正在努力理解std::bind。我已经编写了以下程序 #include <memory> #inc
std::bind
。我已经编写了以下程序
#include <memory>
#include <functional>
#include <iostream>
#include <algorithm>
using namespace std::placeholders;
int add(int first,int second)
{
return first + second;
}
bool divisible(int num, int den)
{
if (num %den == 0)
return true;
return false;
}
int approach_1()
{
int arr[10] = {1,20,13,4,5,6,10,28,19,15};
int count = 0;
for (int i = 0; i < sizeof(arr)/sizeof(int); i++)
{
if(divisible(arr[i],5))
count++;
}
}
int approach_2()
{
int arr[10] = {1,20,13,4,5,6,10,28,19,15};
return count_if(arr,arr + sizeof(arr)/sizeof(int), std::bind(divisible,_1,5));
}
std::placeholder
是在C++11中添加的,因此对于g++而言,必须使用启用C++11或更高版本的编译器开关;例如-std=c++11
或-std=c++14
通常,当您试图使用C++11特性而不使用正确的开关时,g++会在错误消息中提到;但不幸的是,这似乎不是其中一次。@M.M-这确实有效,谢谢。
std_bind.cpp:6:22: error: ‘placeholders’ is not a namespace-name
using namespace std::placeholders;
^
std_bind.cpp:6:34: error: expected namespace-name before ‘;’ token
using namespace std::placeholders;
^
std_bind.cpp: In function ‘int approach_2()’:
std_bind.cpp:36:54: error: ‘bind’ is not a member of ‘std’
return count_if(arr,arr + sizeof(arr)/sizeof(int), std::bind(divisible,_1,5));
^
std_bind.cpp:36:74: error: ‘_1’ was not declared in this scope
return count_if(arr,arr + sizeof(arr)/sizeof(int), std::bind(divisible,_1,5));
^
std_bind.cpp:36:79: error: ‘count_if’ was not declared in this scope
return count_if(arr,arr + sizeof(arr)/sizeof(int), std::bind(divisible,_1,5));
^
std_bind.cpp:36:79: note: suggested alternative:
In file included from /usr/include/c++/4.9/algorithm:62:0,
from std_bind.cpp:4:
/usr/include/c++/4.9/bits/stl_algo.h:3970:5: note: ‘std::count_if’
count_if(_InputIterator __first, _InputIterator __last, _Predicate __pred)
^