# Problem calling string parse + eval from C++

**URL:** <https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701>\
**Category:** General Usage\
**Tags:** embedding, metaprogramming\
**Created:** [April 30, 2019, 3:09pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701 "2019-04-30T15:09:04Z")\
**Posts on this page:** 14\
**Page:** 1

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**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [April 30, 2019, 3:09pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/1 "2019-04-30T15:09:04Z")

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> s::String|\>Meta.parse|\>eval seems good in REPL,but not works when calling julia from C++. I have no idea why?  
> but the following works in both:  
> getproperty(Main,s::String|\>Symbol) return DataType

define.jl:

```julia
#!/usr/bin/env julia

function define(a::String)
	exp="mutable struct $a\n\t a::Int\nend\n"
	println("before")
	exp|>Meta.parse|>eval
	println("after")
end

```

define.cpp:

```julia
#define JULIA_ENABLE_THREADING
#include <julia.h>
JULIA_DEFINE_FAST_TLS()

int main(int argc,char *argv[])
{
	jl_init();
	jl_eval_string("Base.MainInclude.include(\"define.jl\")");

	jl_function_t *obj=jl_get_function(jl_main_module, "define");
	jl_value_t *a=jl_cstr_to_string("Thost");
	JL_GC_PUSH1(&a);
	jl_call1(obj,a);
	JL_GC_POP();
};

```

built by:

```julia
g++ -o define -fPIC -I$JULIA_DIR/include/julia -L$JULIA-DIR/lib define.cpp -ljulia /usr/lib/libstdc++.so.6

```

when run the define.jl in REPL, “before" and “after” are both output.  
when run the cpp by ./define, only “before” is output.

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<div class="post-metadata">

**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [May 14, 2019, 2:42pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/2 "2019-05-14T14:42:29Z")

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It seems that eval does not work, when calling from C++ using jl\_call. Anyone knows why?

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 14, 2019, 2:50pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/3 "2019-05-14T14:50:27Z")

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> [@alligatorme](#):
>
> It seems that eval does not work

“It does not work” questions rarely garner a useful response. What, specifically, did you try, what was the expected output, and what output did you get?

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**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [May 15, 2019, 2:18am UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/4 "2019-05-15T02:18:12Z")

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😅Thank you! the following is my code:

```julia
function define(a::String)
	isdefined(Main,a|>Symbol) && return a
	exp="mutable struct $a\n"
	for (k,v) in tfull[a]
		line=isdigit.([k...])|>all ? "$v::NTuple{$k,Cchar}" : "$v::$(get(typet,k) do; define(k); end)"
		exp*="\t"*line*"\n"
	end
	exp*"end\n"|>Meta.parse|>eval
	println(a)
	a
end

```

The function dynamically and recursively defines a nested struct.  
when called from C++ using jl\_call, I cannot get the output of “println(a)", the problem came from eval in previous line. But the function is fine when runing in REPL. Actually, it is the same problem with that in the top post.

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<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 15, 2019, 11:55am UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/5 "2019-05-15T11:55:38Z")

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> [@alligatorme](#):
>
> I cannot get the output of `“println(a)"`

So is the problem with `println`? That is, if you just replace the whole function with `println(a)`, do you still not get output when you call it with `jl_call`? (When you say “I cannot get the output”, do you mean that it runs but produces no output, or that it gives some kind of error?)

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<div class="post-metadata">

**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [May 15, 2019, 12:59pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/6 "2019-05-15T12:59:03Z")

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After remove the rest code, the **println(a)** get right output, so I think it is the problem of eval.

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**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [May 15, 2019, 2:05pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/7 "2019-05-15T14:05:03Z")

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I’m afraid that without being more clear about what you’re doing—what C++ code are you running?—and what happens—what is the exact output?—you are not going to get much more help.

Also, metaprogramming with strings like this is very much not recommended. It’s the only option in many dynamic languages, but Julia has a first class representation of code with which you can do real metaprogramming.

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**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [May 15, 2019, 2:24pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/8 "2019-05-15T14:24:59Z")

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The code in C++ side

```julia
jl_function_t *obj=jl_get_function(jl_main_module, "define");
jl_value_t *a=jl_cstr_to_string("CThost");
JL_GC_PUSH1(&a);
jl_call1(obj,a);
JL_GC_POP();

```

May I have more your advice about the my previous Julia codes about metaprogramming? Thanks

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<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [May 15, 2019, 3:22pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/9 "2019-05-15T15:22:27Z")

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I can’t run any of your example code because it is incomplete. This is what I get when I try to run it:

```julia
julia> define("Foo")
ERROR: UndefVarError: tfull not defined
Stacktrace:
 [1] define(::String) at ./REPL[26]:4
 [2] top-level scope at REPL[28]:1

```

In order to give you more specific advice on how to do metaprogramming correctly, I would have to spend time to try to modify your code so that it works and _then_ rewrite it to use expression metaprogramming instead of string metaprogramming. I’m willing to do the latter but not the former. Similarly, your C++ code is incomplete and not something I can paste into a text file and compile with gcc or clang. In order to diagnose or test it, I would have to reconstruct the rest of the C++ file in order to be able to compile it. That is not something I’m willing to spend my time on, especially since you presumably have a C++ source file that compiles already.

You are asking for help—which people are willing to give, as the many replies here indicate—but by not posting self-contained, complete code, you are creating a situation where those who would help you, not only need to do the work inherent in helping you but _also_ need to do the extra work of turning your descriptions of code and incomplete snippets into something that can be diagnosed or tested or even understood. I’m perfectly willing to help but I am not willing to do that additional and unnecessary work that you could eliminate by posting self-contained examples to ask questions about.

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**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [May 15, 2019, 3:36pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/10 "2019-05-15T15:36:36Z")

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What Stefan is saying is:

> [@Please read: make it easier to help you](https://discourse.julialang.org/t/psa-make-it-easier-to-help-you/14757):
>
> Welcome to the Julia Discourse! We are enthusiastic about helping Julia programmers, both beginner and experienced. This public service announcement (PSA) outlines best practices when asking for help. Following these points makes it easier for us to help you and more likely you’ll get a prompt, useful answer. Keywords are highlighted to make it easier to refer to specific points. Choose a descriptive title that captures the key part of your question, eg “plots with multiple axes” instead of …

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**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [May 16, 2019, 1:33am UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/11 "2019-05-16T01:33:14Z")

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Got it. I modify the top post and past the simplified code and describe the output results.

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**Author:** ![c42f](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/c42f/32/52842_2.png) [@c42f](https://discourse.julialang.org/u/c42f)\
**Post date:** [May 16, 2019, 4:54am UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/12 "2019-05-16T04:54:35Z")

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Your post looks good now: there’s enough information to compile and run the code locally and figure out what happened here. Here’s the cause of this confusing behavior:

1. One thing you’ve already noticed is that `Main.include` is not present in embedded julia (I think this is a bug and someone else does too: [Embedded Julia doesn't know how to include · Issue #28825 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/28825)). For exactly the same reason, `Main.eval` is also not present so your use of `eval` inside `define` will fail, but only in embedded julia.
2. Because `eval` is not present, a `MethodError` is thrown and you never get to see `after` printed.
3. Because you don’t do any error handling on the embedded side (ie, you never call `jl_exception_occurred()`), you didn’t notice the `MethodError`.

The very simplest workaround is to use `Base.eval(Main, Meta.parse(exp))` inside define.

However, as Stefan already mentioned, there’s _much_ better ways to construct expressions in Julia than string munging. For example, in this simple case you could do it by interpolating the struct name into the expression:

```julia
 function define(struct_name::Symbol) 
    @eval mutable struct $struct_name
        a::Int
    end
    getproperty(Main, struct_name) # Look up and return the newly generated type
end

```

Then call as follows:

```julia
define(:Thost)

```

Or from C:

```nohighlight
    jl_function_t *obj = jl_get_function(jl_main_module, "define");
    jl_value_t *struct_name = (jl_value_t*)jl_symbol("Thost");
    JL_GC_PUSH1(&struct_name);
    jl_value_t *struct_type = jl_call1(obj, struct_name);

```

Having written the above, I think it’s worth asking why you want to define types dynamically like this at all? There’s good reasons of course, but if you can avoid it that’s usually even better. (One good reason would be to reflect types into julia which are only known dynamically from the C++ side.)

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**Author:** ![alligatorme](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alligatorme/32/6962_2.png) [@alligatorme](https://discourse.julialang.org/u/alligatorme)\
**Post date:** [May 16, 2019, 1:31pm UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/13 "2019-05-16T13:31:48Z")

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As you said, but more than 400 static structs in C++ side are demanded to reflect to julia, and I don’t wanna make code so long,😅.

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**Author:** ![c42f](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/c42f/32/52842_2.png) [@c42f](https://discourse.julialang.org/u/c42f)\
**Post date:** [May 17, 2019, 5:18am UTC](https://discourse.julialang.org/t/problem-calling-string-parse-eval-from-c/23701/14 "2019-05-17T05:18:56Z")

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Root cause will be fixed in [Ensure Main.eval and Main.include exist in embedded julia by c42f · Pull Request #32062 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/32062)